A box contains 10 reds, 9 green and some blue balls. Find the probability of getting three different coloured balls when 3 balls are drawn from box at random if total number of balls in the box is 36.
- AOption (a)
1/15
- BOption (b)
2/7
- COption (c)
9/14
- DOption (d)
3/14
- EOption (e)
None of these
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Correct answer
3/14
Why this answer is correct
Shortcut Trick
Total balls = 36 | Red = 10, Green = 9, Blue = 36 − (10 + 9) = 17.
Probability of 3 different colors = (10C1 × 9C1 × 17C1) ÷ 36C3.
⇒ (10 × 9 × 17) ÷ [(36 × 35 × 34) ÷ (3 × 2 × 1)] = 1530 ÷ 7140 = 153/714.
⇒ 153/714 = 3/14 (dividing by 51).
∴ The correct answer is 3/14.
Alternate Method
Given: Red balls = 10, Green balls = 9, Total balls = 36.
Step 1: Calculate number of Blue balls.
⇒ Blue balls = 36 − (10 + 9) = 17.
Step 2: Total ways to draw 3 balls (n(S)).
⇒ 36C3 = (36 × 35 × 34) ÷ (3 × 2 × 1) = 6 × 35 × 34 = 7140.
Step 3: Favorable ways (n(E)) to get one of each color.
⇒ 10C1 × 9C1 × 17C1 = 10 × 9 × 17 = 1530.
Step 4: Probability P(E) = n(E) / n(S).
⇒ P(E) = 1530 ÷ 7140 = 153/714 = 3/14.
∴ The correct answer is 3/14.
Additional Information
Combinations Formula
Selection of r items from n is nCr = n! / [r! × (n−r)!]. Useful for drawing items simultaneously.
Probability Basics
P(Event) = (Number of favorable outcomes) ÷ (Total number of outcomes). Range is always 0 to 1.
AND vs OR Rule
In probability, AND signifies multiplication (independent events), while OR signifies addition (mutually exclusive events).
