The average of 9 numbers is 15. If the average of the first five numbers is 13 and the average of the last five numbers is 17, then what is the fifth number?

Correct option is C

Given:
Average of 9 numbers = 15
Average of first five numbers = 13
Average of last five numbers = 17
Formula Used:
​$$\text{Average} = \frac{\text{Sum of numbers}}{\text{Number of numbers}}$$​​
Solution:
Sum of 9 numbers = 9 × 15 = 135
=> Sum of first five numbers = 5 ×  13 = 65
=> Sum of last five numbers = 5 ×  17 = 85
=> So, the fifth number = (Sum of first five numbers + Sum of last five numbers) - Sum of all 9 numbers = (65 + 85) - 135 = 15.