Correct Answer - Option 4 : 55
Given:
Average of 15 numbers = 90
Average of the first 7 numbers = 80
Average of the last 7 numbers = 105
Formula used:
Sum of observations = Average × Number of observations
Calculation:
Sum of all 15 number = 15 × 90 = 1350
Sum of first 7 numbers = 7 × 80 = 560
Sum of last 7 numbers = 7 × 105 = 735
8th number = Sum of all number – (sum of first 7 numbers + sum of last 7 numbers)
⇒ 1350 – (560 + 735)
⇒ 1350 – 1295
⇒ 55
∴ The 8th number is 55