National Defence Academy (NDA) · Topic Collection

Quantitative Aptitude PYQs

Arithmetic, number systems, percentages, ratios, profit and loss, time and work, algebra, geometry, mensuration, and trigonometry.

2520 Total Questions21 Official PapersYears: 2015–2025Search in Quantitative Aptitude
Showing questions 461–480 of 2520Page 24 of 100
NDA-I 2024 (Mathematics) Official Paper (Held On: 21 Apr, 2024). (Previous Year Paper)·Mathematics
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Question 461

National Defence Academy (NDA) 2024• Mathematics• Mathematics

A bag contains 5 black and 4 white balls. A man selects two balls at random. What is the probability that both of these are of the same colour?

  1. AOption (a)

    \(\frac{1}{6}\)

  2. BOption (b)

    \(\frac{5}{108}\)

  3. COption (c)

    \(\frac{4}{9}\)

  4. DOption (d)

    \(\frac{5}{18}\)

View answer & explanation

Correct answer

Answer: (c)C.

\(\frac{4}{9}\)

Why this answer is correct

Calculation:

Given, bag contains 5 black and 4 white balls.

A man selects two balls at random.

∴ P(both are of same colour)

= P(first ball is black)P(second ball is black) + P(first ball is white)P(seconf ball is white)

= \(\frac{5}{9}\times\frac{4}{8}+\frac{4}{9}\times\frac{3}{8}\) 

= \(\frac{32}{72}\)

= \(\frac{4}{9}\)

∴ The probability that both of these are of the same colour is \(\frac{4}{9}\).

The correct answer is Option 3.

NDA-I 2024 (Mathematics) Official Paper (Held On: 21 Apr, 2024). (Previous Year Paper)·Mathematics
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Question 462

National Defence Academy (NDA) 2024• Mathematics• Mathematics

If a random variable (x) follows binomial distribution with mean 5 and variance 4, and 523P(X = 3) = λ4λ, then what is the value of λ ?

  1. AOption (a)

    3

  2. BOption (b)

    5

  3. COption (c)

    23

  4. DOption (d)

    25

View answer & explanation

Correct answer

Answer: (c)C.

23

Why this answer is correct

Concept:

Let p be the probability of success and q be the probability of failure of a random variable x of sample size n then 

mean (μ) = np and variance (σ2) = npq where q = 1 - p

Also, P(X = x) = nCxpxqn-x

Explanation:

Here mean

np = 5...(i)

and variance

npq = 4...(ii)

So, \({npq\over np}=\frac45\) ⇒ q = \(\frac45\)

Then  p = 1 - q = 1 - \(\frac45\) = \(\frac15\)

Putting in (i) we get

n = 25

Given 

523P(X = 3) = λ4λ

⇒ 523  25C3(\(\frac15\))3(\(\frac45\))22 = λ4λ

⇒ 523  25C3\(\frac{4^{22}}{5^{25}}\) = λ4λ

⇒ \({25!\over 3!.22!}.\frac1{25}\).422 = λ4λ

⇒ \({25.24.23.22!\over 3.2.1.22!}.\frac1{25}\).422 = λ4λ

⇒ 23 × 4 × 422 = λ4λ

⇒ 23 × 423 = λ4λ

⇒ λ = 23

Option (3) is true.

NDA-I 2024 (Mathematics) Official Paper (Held On: 21 Apr, 2024). (Previous Year Paper)·Mathematics
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Question 463

National Defence Academy (NDA) 2024• Mathematics• Mathematics

From data (-4, 1), (-1, 2), (2, 7) and (3, 1), the regression line of y on x is obtained as y = a + bx, then the value of 2a + 15b ?

  1. AOption (a)

    6

  2. BOption (b)

    11

  3. COption (c)

    17

  4. DOption (d)

    21

View answer & explanation

Correct answer

Answer: (b)B.

11

Why this answer is correct

Concept:

The regressing line of y on x is given by y - y̅ = byx(x - x̅), where

\(b_{yx}=\frac{n\sum xy-\sum x\sum y}{n\sum x^2-(\sum x)^2}\)

The regressing line of x on y is given by x - x̅ = bxy(y - y̅), where

\({b_{xy}} = \frac{{n\sum xy - \left( {\sum x} \right)\left( {\sum y} \right)}}{{n\sum {y^2} - {{\left( {\sum y} \right)}^2}}}\)

Calculation:

xyxyx2y2
-41-4161
-12-214
2714449
31391
∑x = 0∑y = 11∑xy = 11∑x2 = 30∑y2 = 55

Here, n = 4

∴ \(b_{yx}=\frac{n\sum xy-\sum x\sum y}{n\sum x^2-(\sum x)^2}\)

= \(\frac{4(11)-0\cdot11}{4\cdot30-0^2}\)

= \(\frac{11}{30}\)

x̅ = \(\frac{\sum x}{n}\) = 0

y̅ = \(\frac{\sum y}{n}\) = \(\frac{11}{4}\) = 2.75

∴ Required line, y - y̅ = byx(x - x̅)

⇒ y - 2.75 = \(\frac{11}{30}\)(x - 0)

⇒ y = 2.75 + \(\frac{11}{30}x\)

Comparing with y = a + bx, a = 2.75 and b = \(\frac{11}{30}\)

⇒ 2a + 15b = 5.5 + 5.5 = 11

∴ The value of 2a + 15b is 11.

The correct answer is Option 2.

NDA-I 2024 (Mathematics) Official Paper (Held On: 21 Apr, 2024). (Previous Year Paper)·Mathematics
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Question 464

National Defence Academy (NDA) 2024• Mathematics• Mathematics

Let x + 2y + 1 = 0 and 2x + 3y + 4 = 0 are two lines of regression computed from some bivariate data. If θ is the acute angle between them, then what is the value of 488 tan3θ ?

  1. AOption (a)

    191

  2. BOption (b)

    161

  3. COption (c)

    131

  4. DOption (d)

    121

View answer & explanation

Correct answer

Answer: (a)A.

191

Why this answer is correct

Given:

Let x + 2y + 1 = 0 and 2x + 3y + 4 = 0 be two lines of regression. If θ is the acute angle between them.

Concept Used:

The angle θ between two lines given by \(a_1x + b_1y + c_1 = 0\) and \(a_2x + b_2y + c_2 = 0\) is given by:

\(\tan θ = \left|\frac{m_1 - m_2}{1 + m_1 m_2}\right|\)

Where m1 and m2 are the slopes of the lines.

Explanation:

For line x + 2y + 1 = 0 :

⇒ \(y = -\frac{1}{2}x - \frac{1}{2} \implies m_1 = -\frac{1}{2}\)

For line 2x + 3y + 4 = 0 :

y = \(-\frac{2}{3}x - \frac{4}{3} \implies m_2 = -\frac{2}{3}\)

\(\tan θ = \left|\frac{-\frac{1}{2} + \frac{2}{3}}{1 + \left(-\frac{1}{2}\right)\left(-\frac{2}{3}\right)}\right|\)

= \(\left|\frac{-\frac{1}{2} + \frac{2}{3}}{1 + \frac{1}{3}}\right| = \left|\frac{\frac{-3+4}{6}}{\frac{4}{3}}\right| = \left|\frac{\frac{1}{6}}{\frac{4}{3}}\right| = \left|\frac{1}{8}\right| = \frac{1}{8}\)  

Using the triple angle formula for tangent 

\(\tan(3θ) = \frac{3\tanθ - \tan^3θ}{1 - 3\tan^2θ}\)

Substitute tan θ  = 1/8 

\(\tan(3θ) = \frac{3 \cdot \frac{1}{8} - \left(\frac{1}{8}\right)^3}{1 - 3 \left(\frac{1}{8}\right)^2}\)

= \(\frac{\frac{3}{8} - \frac{1}{512}}{1 - \frac{3}{64}}\) = \(\frac{191}{64} \times \frac{8}{61}\) = \(\frac{191}{8} \times \frac{1}{61}\)

Now we multiply this value by 488:

488 tan(3θ) = 488 x  \(\frac{191}{8} \times \frac{1}{61}\) = 191

Thus, the value of 488 tan(3θ) is 191.

NDA-I 2024 (Mathematics) Official Paper (Held On: 21 Apr, 2024). (Previous Year Paper)·Mathematics
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Question 465

National Defence Academy (NDA) 2024• Mathematics• Mathematics

If two random variables X and Y are connected by relation \(\frac{2 X-3 Y}{5 X+4 Y}=4\) and X follows Binomial distribution with parameters n = 10 and \(p=\frac{1}{2}\), then what is the variance of Y ?

  1. AOption (a)

    \(\frac{810}{361}\)

  2. BOption (b)

    \(\frac{9}{19}\)

  3. COption (c)

    \(\frac{21}{361}\)

  4. DOption (d)

    \(\frac{121}{361}\)

View answer & explanation

Correct answer

Answer: (a)A.

\(\frac{810}{361}\)

Why this answer is correct

Given:

If two random variables X and Y are connected by the relation

\(\frac{2X - 3Y}{5X + 4Y} = 4\) and X follows a Binomial distribution with parameters

n = 10 and \(p = \frac{1}{2} \)

Concept Used:

For Binomial distribution X ∼ B(n, p) ,

E(X) = np and Var(X) = np(1-p)

Explanation:

Given relation: 

\(\frac{2X - 3Y}{5X + 4Y} = 4 \)

⇒ \( 2X - 3Y = 4(5X + 4Y)\)

⇒ \(2X - 3Y = 20X + 16Y\)

⇒ -18X = 19Y

⇒ Y = \(\frac{-18X}{19}\)

Since X ∼ B(10, 1/2) :

E(X) = \(10 \cdot \frac{1}{2} = 5\)

Var(X) = \(10 \cdot \frac{1}{2} \cdot \frac{1}{2} = \frac{5}{2}\)

Now, finding Var(Y) :

Y = \(\frac{-18X}{19}\)

The variance of Y :

\(\text{Var}(Y) = \left(\frac{-18}{19}\right)^2 \text{Var}(X) = \frac{324}{361} \cdot \frac52 = \frac{810}{361}\)

Thus, the variance of Y is 810/361.

NDA-I 2024 (Mathematics) Official Paper (Held On: 21 Apr, 2024). (Previous Year Paper)·Mathematics
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Question 466

National Defence Academy (NDA) 2024• Mathematics• Mathematics

If a, b, c are in HP, then what is \(\frac{1}{b-a}+\frac{1}{b-c}\) equal to ?

  1. \(\frac{2}{b}\) 2. \(\frac{1}{a}+\frac{1}{c}\) 3. \(\frac{1}{2}\left(\frac{1}{a}+\frac{1}{b}+\frac{1}{c}\right)\)

Select the correct answer using the code given below :

  1. AOption (a)

    only 1 and 2

  2. BOption (b)

    only 2

  3. COption (c)

    only 3

  4. DOption (d)

    1, 2 and 3

View answer & explanation

Correct answer

Answer: (a)A.

only 1 and 2

Why this answer is correct

Explanation:

a, b, c are in HP so

\({2\over b}=\frac1a+\frac1c\)...(i)

⇒ \({2\over b}=\frac{a+c}{ac}\)

⇒ b = \(2ac\over a+c\)....(ii)

∴ \(\frac{1}{b-a}+\frac{1}{b-c}\)

= \(\frac{2b-(c+a)}{(b-a)(b-c)}\)

= \(\frac{2b-(c+a)}{b^2-(a+c)b+ac}\)

= \(\frac{2b-{2ac\over b}}{b^2-2ac+ac}\)

= \(2b^2-2ac\over b(b^2-ac)\) = \(2\over b\) = \(\frac1a+\frac1c\)

1 and 2 both are correct.

NDA-I 2024 (Mathematics) Official Paper (Held On: 21 Apr, 2024). (Previous Year Paper)·Mathematics
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Question 467

National Defence Academy (NDA) 2024• Mathematics• Mathematics

An edible oil is sold at the rates 150, 200, 250, 300 rupees per litre in four consecutive years. Assuming that an equal amount of money is spent on oil by a family in every year during these years, what is the average price of oil in rupees (approximately) per litre?

  1. AOption (a)

    210

  2. BOption (b)

    220

  3. COption (c)

    230

  4. DOption (d)

    240

View answer & explanation

Correct answer

Answer: (a)A.

210

Why this answer is correct

Calculation:

Given the rates are 150, 200, 250, and 300 rupees per litre, the formula for the harmonic mean (H) for (n) values is:

\(H = \frac{n}{\frac{1}{x_1} + \frac{1}{x_2} + \frac{1}{x_3} + \frac{1}{x_4}}\)

where x1, x2 , x3, and x4 are the given rates.

Substituting the given values:

\(H = \frac{4}{\frac{1}{150} + \frac{1}{200} + \frac{1}{250} + \frac{1}{300}}\)

First, compute the sum of the reciprocals of the rates:

1/150 = 0.006667

1/200 = 0.005

1/250 = 0.004

1/300 = 0.003333

Sum these values:

0.006667 + 0.005 + 0.004 + 0.003333 = 0.019

Now, calculate the harmonic mean:

H = 4 / 0.019 ≈ 210.526

Rounded to the nearest whole number, the average price of oil per litre is approximately 210.

NDA-I 2024 (Mathematics) Official Paper (Held On: 21 Apr, 2024). (Previous Year Paper)·Mathematics
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Question 468

National Defence Academy (NDA) 2024• Mathematics• Mathematics

If the letters of the word "TIRUPATI" are written down at random, then what is the probability that both Ts are always consecutive?

  1. AOption (a)

    \(\frac{1}{2}\)

  2. BOption (b)

    \(\frac{1}{4}\)

  3. COption (c)

    \(\frac{1}{7}\)

  4. DOption (d)

    \(\frac{1}{14}\)

View answer & explanation

Correct answer

Answer: (b)B.

\(\frac{1}{4}\)

Why this answer is correct

Explanation -

First, let's calculate the total number of ways to arrange the letters in "TIRUPATI."

The word "TIRUPATI" consists of 8 letters where the letter T is repeated twice.

The total number of permutations of the letters is given by: \(\frac{8!}{2!}\)

where 8! is the factorial of 8, and 2! accounts for the repetition of the letter T.

8! = 40320 and 2! = 2

⇒ \(\frac{8!}{2!} = \frac{40320}{2} = 20160\)

Next, we calculate the number of ways to arrange the letters such that the two Ts are always consecutive.

We can treat the two Ts as a single "super T" unit. This reduces the problem to arranging 7 units: (TT), I, R, U, P, A, I.

The number of ways to arrange these 7 units is \(\frac{7!}{1!}\)

where 7! is the factorial of 7, and 1! accounts for the "super T" being treated as a single unit.

7! = 5040

Since there are no repetitions other than the original T, we don't need to divide by any other factorials.

Therefore, the probability that the two Ts are always consecutive is the ratio of the number of favorable outcomes to the total number of outcomes:

5040 / 20160 = 1/4

So, the correct answer is 1/4.

NDA-I 2024 (Mathematics) Official Paper (Held On: 21 Apr, 2024). (Previous Year Paper)·Mathematics
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Question 469

National Defence Academy (NDA) 2024• Mathematics• Mathematics

Let m = 77n. The index n is given a positive integral value at random. What is the probability that the value of m will have 1 in the units place?

  1. AOption (a)

    \(\frac{1}{2}\)

  2. BOption (b)

    \(\frac{1}{3}\)

  3. COption (c)

    \(\frac{1}{4}\)

  4. DOption (d)

    \(\frac{1}{n}\)

View answer & explanation

Correct answer

Answer: (c)C.

\(\frac{1}{4}\)

Why this answer is correct

Given:

Let m = 77n. The index n is given a positive integral value at random.

Concept Used:

To determine the units place of a number resulting from a multiplication, we only need to consider the units place of the numbers being multiplied.

Let's analyze the units place of 77 raised to various powers:

  • 77 has a units place of 7
  • 772 = 5929 has a units place of 9
  • 773 = 456533 has a units place of 3
  • 774 = 35153041 has a units place of 1

We observe that the units digit of 77n repeats every 4 cycles: 7, 9, 3, 1.

Calculation:

We need to find the probability that the units place of m (i.e., 77n) will be 1.

From the above pattern, we see that the units place is 1 whenever n is a multiple of 4 (i.e., n = 4k for some integer k).

Given that n is a positive integer, the probability that n is a multiple of 4 among all positive integers is:

Number of favourable outcomes (multiples of 4) = 1, 2, 3, 4, ...

Total number of outcomes = n

Probability  = 1/4 

∴ The probability that the value of m will have 1 in the units place is 1/4.

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Question 470

National Defence Academy (NDA) 2024• Mathematics• Mathematics

Three different numbers are selected at random from the first 15 natural numbers. What is the probability that the product of two of the numbers is equal to third number?

  1. AOption (a)

    \(\frac{1}{91}\)

  2. BOption (b)

    \(\frac{2}{455}\)

  3. COption (c)

    \(\frac{1}{65}\)

  4. DOption (d)

    \(\frac{6}{455}\)

  5. EOption (e)

    1/67

View answer & explanation

Correct answer

Answer: (c)C.

\(\frac{1}{65}\)

Why this answer is correct

Given:

Three different numbers are selected at random from the first 15 natural numbers.

Formula Used:

Probability = \(\frac{Number\ of\ Favourable\ Outcomes}{Total\ Number\ of\ Outcomes}\)

Calculation:

Total number of outcomes = \(\binom{15}{3} = \frac{15!}{3!(15-3)!} = \frac{15 \times 14 \times 13}{3 \times 2 \times 1} = 455\)  

Let's find the number of favorable outcomes where the product of first two of the numbers is equal to the third number.

All the numbers are distinct numbers from 1 to 15.

The possible sets are:

  • (1, 2, 2) - Invalid since numbers must be distinct.
  • (1, 3, 3) - Invalid since numbers must be distinct.
  • (1, 4, 4) - Invalid since numbers must be distinct.
  • (1, 5, 5) - Invalid since numbers must be distinct.
  • (1, 6, 6) - Invalid since numbers must be distinct.
  • (1, 7, 7) - Invalid since numbers must be distinct.
  • (1, 8, 8) - Invalid since numbers must be distinct.
  • (1, 9, 9) - Invalid since numbers must be distinct.
  • (1, 10, 10) - Invalid since numbers must be distinct.
  • (1, 11, 11) - Invalid since numbers must be distinct.
  • (1, 12, 12) - Invalid since numbers must be distinct.
  • (1, 13, 13) - Invalid since numbers must be distinct.
  • (1, 14, 14) - Invalid since numbers must be distinct.
  • (1, 15, 15) - Invalid since numbers must be distinct.
  • (2, 3, 6) - Valid
  • (3, 4, 12) - Valid
  • (2, 4, 8) - Valid
  • (3, 5, 15) - Valid
  • (2, 5, 10) - Valid
  • (2, 6, 12) - Valid
  • (2, 7, 14) - Valid

 

Total number of favorable outcomes = 7

Therefore, the probability = 7/ 455 = 1/65

∴ The probability that the product of two of the numbers is equal to the third number is 1/65.

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Question 471

National Defence Academy (NDA) 2024• Mathematics• Mathematics

Consider the following for the items that follow:

Let A and B be two events such that P(A ∪ B) ≥ 0.75 and 0.125 ≤ P(A ∩ B) ≤ 0.375.

What is the minimum value of P(A) + P(B) ?

  1. AOption (a)

    0.625

  2. BOption (b)

    0.750

  3. COption (c)

    0.825

  4. DOption (d)

    0.875

View answer & explanation

Correct answer

Answer: (d)D.

0.875

Why this answer is correct

Concept:

P(A ∪ B) = P(A) + P(B) - P(A ∩ B)

Calculation:

Given, P(A ∪ B) ≥ 0.75 and 0.125 ≤ P(A ∩ B) ≤ 0.375.

Now, P(A) + P(B) - P(A ∩ B) = P(A ∪ B)

⇒ P(A) + P(B) - P(A ∩ B) ≥ 0.75

To find the minimum value of P(A) + P(B), we need the smallest possible value for P(A ∩ B), which is 0.125.

⇒ P(A) + P(B) ≥ 0.75 + 0.125

⇒ P(A) + P(B) ≥ 0.875

∴ Minimum value of P(A) + P(B) is 0.875

The correct answer is Option 4.

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Question 472

National Defence Academy (NDA) 2024• Mathematics• Mathematics

Consider the following for the items that follow:

Let A and B be two events such that P(A ∪ B) ≥ 0.75 and 0.125 ≤ P(A ∩ B) ≤ 0.375.

What is the maximum value of P(A) + P(B) ?

  1. AOption (a)

    0.75

  2. BOption (b)

    1.125

  3. COption (c)

    1.375

  4. DOption (d)

    1.625

View answer & explanation

Correct answer

Answer: (c)C.

1.375

Why this answer is correct

Concept:

P(A ∪ B) = P(A) + P(B) - P(A ∩ B)

Calculation:

Given, P(A ∪ B) ≥ 0.75 and 0.125 ≤ P(A ∩ B) ≤ 0.375.

Now, P(A) + P(B) - P(A ∩ B) = P(A ∪ B)

⇒ P(A) + P(B) - P(A ∩ B) ≥ 0.75

To find the maximum value of P(A) + P(B), we need the largest possible value for P(A ∩ B), which is 0.375.

⇒ P(A) + P(B) ≤  0.75 + 0.375

⇒ P(A) + P(B) ≤ 1.125

∴ Maximum value of P(A) + P(B) is 1.125.

The correct answer is Option 2.

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Question 473

National Defence Academy (NDA) 2024• Mathematics• Mathematics

Consider the following for the items that follow:

A, B and C are three events such that P(A) = 0.6, P(B) = 0.4, P(C) = 0.5, P(A ∪ B) = 0.8, P(A ∩ C) = 0.3 and P(A ∩ B ∩ C) = 0.2 and P(A ∪ B ∪ C) ≥ 0.85.

What is the minimum value of P(B ∩ C) ?

  1. AOption (a)

    0.1

  2. BOption (b)

    0.2

  3. COption (c)

    0.35

  4. DOption (d)

    0.45

View answer & explanation

Correct answer

Answer: (b)B.

0.2

Why this answer is correct

Concept:

P(A ∪ B ∪ C) = P(A) + P(B) + P(C) - P(A ∩ B) - P(B ∩ C) - P(A ∩ C) + P(A ∩ B ∩ C)

Calculation:

Given, P(A) = 0.6, P(B) = 0.4, P(C) = 0.5

P(A ∪ B) = 0.8, P(A ∩ C) = 0.3 and P(A ∩ B ∩ C) = 0.2 and P(A ∪ B ∪ C) ≥ 0.85

Now, P(A ∪ B) = 0.8

⇒ P(A) + P(B) - P(A ∩ B) = 0.8

⇒ 0.6 + 0.4 - P(A ∩ B) = 0.8

⇒ P(A ∩ B) = 0.2 ⋯ (i)

∴ P(A ∪ B ∪ C) = P(A) + P(B) + P(C) - P(A ∩ B) - P(B ∩ C) - P(A ∩ C) + P(A ∩ B ∩ C)

According to the question, P(A ∪ B ∪ C) ≥ 0.85

⇒ P(A) + P(B) + P(C) - P(A ∩ B) - P(B ∩ C) - P(A ∩ C) + P(A ∩ B ∩ C) ≥ 0.85

⇒ 0.6 + 0.4 + 0.5 - 0.2 - P(B ∩ C) - 0.3 + 0.2 ≥ 0.85

⇒ 1.2 - P(B ∩ C) ≥ 0.85

⇒ - P(B ∩ C) ≥ - 0.35

⇒ P(B ∩ C) ≤ 0.35

So maximum value of P(B ∩ C) is 0.35. But we need the minimum value.

Let’s consider what values we cannot go below.

We are also given:

P(A ∩ C) = 0.3

P(A ∩ B ∩ C) = 0.2

So the part of C that overlaps with both A and B is already 0.2.

So, P(B ∩ C) ≥ P(A ∩ B ∩ C) = 0.2

Hence, the Minimum value of P(B ∩ C) is 0.2​

The correct answer is Option 2.

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Question 474

National Defence Academy (NDA) 2024• Mathematics• Mathematics

Consider the following for the items that follow:

A, B and C are three events such that P(A) = 0.6, P(B) = 0.4, P(C) = 0.5, P(A ∪ B) = 0.8, P(A ∩ C) = 0.3 and P(A ∩ B ∩ C) = 0.2 and P(A ∪ B ∪ C) ≥ 0.85.

What is the maximum value of P(B ∩ C) ?

  1. AOption (a)

    0.1

  2. BOption (b)

    0.2

  3. COption (c)

    0.35

  4. DOption (d)

    0.45 

View answer & explanation

Correct answer

Answer: (c)C.

0.35

Why this answer is correct

Concept:

P(A ∪ B ∪ C) = P(A) + P(B) + P(C) - P(A ∩ B) - P(B ∩ C) - P(A ∩ C) + P(A ∩ B ∩ C)

Calculation:

Given, P(A) = 0.6, P(B) = 0.4, P(C) = 0.5

P(A ∪ B) = 0.8, P(A ∩ C) = 0.3 and P(A ∩ B ∩ C) = 0.2 and P(A ∪ B ∪ C) ≥ 0.85

Now, P(A ∪ B) = 0.8

⇒ P(A) + P(B) - P(A ∩ B) = 0.8

⇒ 0.6 + 0.4 - P(A ∩ B) = 0.8

⇒ P(A ∩ B) = 0.2 ⋯ (i)

∴ P(A ∪ B ∪ C) = P(A) + P(B) + P(C) - P(A ∩ B) - P(B ∩ C) - P(A ∩ C) + P(A ∩ B ∩ C)

According to the question, P(A ∪ B ∪ C) ≥ 0.85

⇒ P(A) + P(B) + P(C) - P(A ∩ B) - P(B ∩ C) - P(A ∩ C) + P(A ∩ B ∩ C) ≥ 0.85

⇒ 0.6 + 0.4 + 0.5 - 0.2 - P(B ∩ C) - 0.3 + 0.2 ≥ 0.85

⇒ 1.2 - P(B ∩ C) ≥ 0.85

⇒ - P(B ∩ C) ≥ - 0.35

⇒ P(B ∩ C) ≤ 0.35

∴ The maximum value of P(B ∩ C) is 0.35.

The correct answer is Option 3.

NDA-I 2024 (Mathematics) Official Paper (Held On: 21 Apr, 2024). (Previous Year Paper)·Mathematics
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Question 475

National Defence Academy (NDA) 2024• Mathematics• Mathematics

Consider the following for the items that follow :

An unbiased coin is tossed n times. The probability of getting at least one tail is p and the probability of at least two tails is q and \(p-q=\frac{5}{32}\).

What is the value of n ?

  1. AOption (a)

    4

  2. BOption (b)

    5

  3. COption (c)

    6

  4. DOption (d)

    7

View answer & explanation

Correct answer

Answer: (b)B.

5

Why this answer is correct

Given: 

An unbiased coin is tossed n times. The probability of getting at least one tail is p and the probability of getting at least two tails is q . It is given that p - q = 5/32

Explanation -

Probability of at least one tail p:

p = 1 - Probability of getting no tail

The probability of getting no tail in n tosses = \( \left( \frac{1}{2} \right)^n \)

Therefore, p = 1 - \( \left( \frac{1}{2} \right)^n \)  

Probability of at least two tails q:

q = 1 - Probability of getting no tail + Probability of getting exactly one tail

The probability of getting exactly one tail in n tosses is 

\(\binom{n}{1} \left( \frac{1}{2} \right)^1 \left( \frac{1}{2} \right)^{n-1} = n \left( \frac{1}{2} \right)^n\)

Therefore,

\(q = 1 - \left( \frac{1}{2} \right)^n - n \left( \frac{1}{2} \right)^n = 1 - \left( \frac{1+n}{2^n} \right)\)

It is given that p - q = 5/32

\(\left(1 - \left( \frac{1}{2} \right)^n\right) - \left(1 - \left( \frac{1+n}{2^n} \right)\right) = \frac{5}{32}\)

Simplifying this, we get:

\( \left(1 - \left( \frac{1}{2} \right)^n\right) - \left(1 - \left( \frac{1+n}{2^n} \right)\right) = \frac{5}{32}\)

\(\frac{1+n}{2^n} - \left( \frac{1}{2} \right)^n = \frac{5}{32}\)

\(\frac{1+n - 1}{2^n} = \frac{5}{32}\)

\(\frac{n}{2^n} = \frac{5}{32}\)

This implies that:

n = 5

Thus, the value of n is 5.

NDA-I 2024 (Mathematics) Official Paper (Held On: 21 Apr, 2024). (Previous Year Paper)·Mathematics
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Question 476

National Defence Academy (NDA) 2024• Mathematics• Mathematics

Consider the following for the items that follow :

An unbiased coin is tossed n times. The probability of getting at least one tail is p and the probability of at least two tails is q and \(p-q=\frac{5}{32}\).

What is the value of p + q ?

  1. AOption (a)

    \(\frac{57}{32}\)

  2. BOption (b)

    \(\frac{53}{32}\)

  3. COption (c)

    \(\frac{51}{32}\)

  4. DOption (d)

    1 

View answer & explanation

Correct answer

Answer: (a)A.

\(\frac{57}{32}\)

Why this answer is correct

Explanation -

Given:

p = \(1 - \left( \frac{1}{2} \right)^5\)  and q = \(1 - \left( \frac{1+5}{2^5} \right)\)

Now 

\(p = 1 - \left( \frac{1}{2} \right)^5 = 1 - \frac{1}{32} = \frac{31}{32} \)   

And 

\(q = 1 - \left( \frac{1+5}{2^5} \right) = 1 - \left( \frac{6}{32} \right) = 1 - \frac{3}{16} = \frac{13}{16} \)   

⇒ \(p + q = \frac{31}{32} + \frac{13}{16} \)

⇒ p + q = 57/32

Thus, the value of p + q is 57/32.

NDA-I 2024 (Mathematics) Official Paper (Held On: 21 Apr, 2024). (Previous Year Paper)·Mathematics
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Question 477

National Defence Academy (NDA) 2024• Mathematics• Mathematics

Consider the following for the items that follow :

xi123...n
fi12-12-2...2-(n-1)

What is \(\sum_i^n x_i f_i\) equal to ?

  1. AOption (a)

    \(\frac{2^{n+1}-n+2}{2^{n-1}}\)

  2. BOption (b)

    \(\frac{2^{n+1}-n-2}{2^{n-1}}\)

  3. COption (c)

    \(\frac{2^{n+1}+n+2}{2^{n-1}}\)

  4. DOption (d)

    \(\frac{2^{n+1}-n-2}{2^n}\)

View answer & explanation

Correct answer

Answer: (b)B.

\(\frac{2^{n+1}-n-2}{2^{n-1}}\)

Why this answer is correct

Concept:

Sum of a n terms GP a, ar, ar2, ⋯ , arn-1 is given by:

S = \(\frac{a(r^n-1)}{r-1}\), if r > 1

else, S = \(\frac{a(1-r^n)}{1-r}\), if r < 1

Calculation:

Given, 

xi123...n
fi12-12-2...2-(n-1)

 

∴ \(\sum_i^n x_i f_i\)

= \(\frac{1}{1}+\frac{2}{2}+\frac{3}{2^2}+⋯+\frac{n}{2^{n-1}}\)

Let S = \(\frac{1}{1}+\frac{2}{2}+\frac{3}{2^2}+⋯+\frac{n}{2^{n-1}}\) ⋯ (i)

⇒ \(\frac{1}{2}\)S =       \(\frac{1}{2}+\frac{2}{2^2}+\frac{3}{2^3}+⋯+\frac{n}{2^n}\)⋯ (ii)

(i) - (ii)

⇒ \(\frac{1}{2}\)S = \(1+\frac{1}{2}+\frac{1}{2^2}+⋯+\frac{1}{2^{n-1}}-\frac{n}{2^n}\)

⇒ \(\frac{1}{2}\)S = \(\frac{1(1-\frac{1}{2^n})}{1-\frac{1}{2}}-\frac{n}{2^n}\)

⇒ \(\frac{1}{2}\)S = \(2(1-\frac{1}{2^n})-\frac{n}{2^n}\)

⇒ \(\frac{1}{2}\)S = \(2-\frac{1}{2^{n-1}}-\frac{n}{2^n}\)

⇒ S = \(2^2-\frac{1}{2^{n-2}}-\frac{n}{2^{n-1}}\)

⇒ S = \(\frac{2^n-1}{2^{n-2}}-\frac{n}{2^{n-1}}\)

⇒ S = \(\frac{2^{n+1}-2}{2^{n-1}}-\frac{n}{2^{n-1}}\)

⇒ S = \(\frac{2^{n+1}-n-2}{2^{n-1}}\)

∴ The value of \(\sum_i^n x_i f_i\) is equal to \(\frac{2^{n+1}-n-2}{2^{n-1}}\).

The correct answer is Option 2.

NDA-I 2024 (Mathematics) Official Paper (Held On: 21 Apr, 2024). (Previous Year Paper)·Mathematics
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Question 478

National Defence Academy (NDA) 2024• Mathematics• Mathematics

Consider the following for the items that follow :

xi123...n
fi12-12-2...2-(n-1)

What is the mean of the distribution ?

  1. AOption (a)

    \(\frac{2^{n+1}-n+2}{2^n-1}\)

  2. BOption (b)

    \(\frac{2^{n+1}-n-2}{2^{n-1}}\)

  3. COption (c)

    \(\frac{2^{n+1}-n-2}{2^n-1}\)

  4. DOption (d)

    \(\frac{2^{n+1}-n+2}{2^n}\)

View answer & explanation

Correct answer

Answer: (c)C.

\(\frac{2^{n+1}-n-2}{2^n-1}\)

Why this answer is correct

Concept:

The mean of a distribution is given by, x̅ = \(\frac{\sum_i^n x_i f_i}{\sum_i^nf_i}\)

Calculation:

Given, 

xi123...n
fi12-12-2...2-(n-1)

 

∴ \(\sum_i^n x_i f_i\)

= \(\frac{1}{1}+\frac{2}{2}+\frac{3}{2^2}+⋯+\frac{n}{2^{n-1}}\)

Let S = \(\frac{1}{1}+\frac{2}{2}+\frac{3}{2^2}+⋯+\frac{n}{2^{n-1}}\) ⋯ (i)

⇒ \(\frac{1}{2}\)S =       \(\frac{1}{2}+\frac{2}{2^2}+\frac{3}{2^3}+⋯+\frac{n}{2^n}\)⋯ (ii)

(i) - (ii)

⇒ \(\frac{1}{2}\)S = \(1+\frac{1}{2}+\frac{1}{2^2}+⋯+\frac{1}{2^{n-1}}-\frac{n}{2^n}\)

⇒ \(\frac{1}{2}\)S = \(\frac{1(1-\frac{1}{2^n})}{1-\frac{1}{2}}-\frac{n}{2^n}\)

⇒ \(\frac{1}{2}\)S = \(2(1-\frac{1}{2^n})-\frac{n}{2^n}\)

⇒ \(\frac{1}{2}\)S = \(2-\frac{1}{2^{n-1}}-\frac{n}{2^n}\)

⇒ S = \(2^2-\frac{1}{2^{n-2}}-\frac{n}{2^{n-1}}\)

⇒ S = \(\frac{2^n-1}{2^{n-2}}-\frac{n}{2^{n-1}}\)

⇒ S = \(\frac{2^{n+1}-2}{2^{n-1}}-\frac{n}{2^{n-1}}\)

⇒ S = \(\frac{2^{n+1}-n-2}{2^{n-1}}\)

∴ \(\sum_i^nf_i\) = \(1+\frac{1}{2}+\frac{1}{2^2}+⋯+\frac{1}{2^{n-1}}\)

= \(\frac{1(1-\frac{1}{2^n})}{1-\frac{1}{2}}\)

= \(2-\frac{1}{2^{n-1}}\)

= \(\frac{2^n-1}{2^{n-1}}\)

⇒ x̅ = \(\frac{\sum_i^n x_i f_i}{\sum_i^nf_i}\) = \(\frac{\frac{2^{n+1}-n-2}{2^{n-1}}}{\frac{2^n-1}{2^{n-1}}}\) = \(\frac{2^{n+1}-n-2}{2^{n}-1}\)

∴ The mean of the distribution is \(\frac{2^{n+1}-n-2}{2^{n}-1}\).

The correct answer is Option 3.

NDA-I 2024 (Mathematics) Official Paper (Held On: 21 Apr, 2024). (Previous Year Paper)·Mathematics
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Question 479

National Defence Academy (NDA) 2024• Mathematics• Mathematics

Consider the following for the items that follow :

The marks obtained by 10 students in a Statistics test are 24, 47, 18, 32, 19, 15, 21, 35, 50 and 41.

What is the mean deviation of the largest five observations ? 

  1. AOption (a)

    4.8

  2. BOption (b)

    5.5

  3. COption (c)

    6

  4. DOption (d)

    7.5

View answer & explanation

Correct answer

Answer: (c)C.

6

Why this answer is correct

Explanation -

The marks obtained by 10 students in a Statistics test are:

24, 47, 18, 32, 19, 15, 21, 35, 50, and 41.

First, we sort these marks in ascending order:

15, 18, 19, 21, 24, 32, 35, 41, 47, 50

The largest five observations are:

32, 35, 41, 47, 50

Mean Deviation of the Largest Five Observations

The Mean:

\(\text{Mean} = \frac{32 + 35 + 41 + 47 + 50}{5} = \frac{205}{5} = 41 \)

The Mean Deviation:

\(\text{Mean Deviation} = \frac{|32 - 41| + |35 - 41| + |41 - 41| + |47 - 41| + |50 - 41|}{5} \)

= \(\frac{9 + 6 + 0 + 6 + 9}{5}\) = 30/5 = 6

The mean deviation of the largest five observations is 6.

NDA-I 2024 (Mathematics) Official Paper (Held On: 21 Apr, 2024). (Previous Year Paper)·Mathematics
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Question 480

National Defence Academy (NDA) 2024• Mathematics• Mathematics

Consider the following for the items that follow :

The marks obtained by 10 students in a Statistics test are 24, 47, 18, 32, 19, 15, 21, 35, 50 and 41.

What is the variance of the largest five observations?

  1. AOption (a)

    14.6

  2. BOption (b)

    21.8

  3. COption (c)

    25.2

  4. DOption (d)

    46.8

View answer & explanation

Correct answer

Answer: (d)D.

46.8

Why this answer is correct

Explanation -

The marks obtained by 10 students in a Statistics test are:

24, 47, 18, 32, 19, 15, 21, 35, 50, and 41.

First, we sort these marks in ascending order:

15, 18, 19, 21, 24, 32, 35, 41, 47, 50

The largest five observations are:

32, 35, 41, 47, 50

Variance of the Largest Five Observations

The Variance:

we already have the mean μ = 41.

The Sum of Squared Deviations:

\(\text{Variance} = \frac{(32 - 41)^2 + (35 - 41)^2 + (41 - 41)^2 + (47 - 41)^2 + (50 - 41)^2}{5} \)

= \(\frac{(-9)^2 + (-6)^2 + (0)^2 + (6)^2 + (9)^2}{5}\)

=\( \frac{81 + 36 + 0 + 36 + 81}{5}\) = 234/5 = 46.8

The variance of the largest five observations is 46.8.

How should I prepare for Quantitative Aptitude in National Defence Academy (NDA)?

Solving previous year questions by topic helps identify recurring concepts, question patterns, and frequently asked sub-topics. Attempt each question, review the step-by-step explanation, and note down key formulas or facts for quick revision before the exam.