Shortcut Trick
For any set of numbers, if every number in the set is increased by a constant k, the new average will also increase by k.
The first sequence starts at a and the second sequence starts at a + 7, meaning each of the 13 consecutive numbers has increased by exactly 7.
Since the initial average is x and the increase per term is 7, the new average is x + 7.
∴ The correct answer is x + 7.
Alternate Method
Given:
Number of terms (n) = 13
First sequence starts with a. Average = x
Second sequence starts with a + 7.
Formula Used:
Average of consecutive numbers = (First term + Last term) ÷ 2
Calculations:
⇒ For the first sequence: a, a+1, a+2, ..., a+12
⇒ Average x = (a + a + 12) ÷ 2
⇒ x = (2a + 12) ÷ 2 = a + 6
⇒ For the second sequence: (a + 7), (a + 8), ..., (a + 7 + 12)
⇒ Last term of second sequence = a + 19
⇒ New Average = ((a + 7) + (a + 19)) ÷ 2
⇒ New Average = (2a + 26) ÷ 2 = a + 13
⇒ Comparing with x: New Average = (a + 6) + 7 = x + 7
∴ The correct answer is x + 7.
Additional Information
Average of Arithmetic Progression
For any sequence in AP, the average is exactly the middle term if the number of terms is odd, or the mean of the two middle terms if even.
Linear Property of Mean
If every observation in a data set is multiplied by k and then increased by c, the new mean becomes (Original Mean × k) + c.
Sum of Natural Numbers
The sum of first n consecutive natural numbers is given by the formula: [n(n + 1)] ÷ 2.