There are 15 numbers. The average of all 15 is 20. The average of the first 9 numbers is 18.
The average of the last 7 numbers is 24. What is the 9th number?
Show answer & explanation
Correct answer: C
Explanation
Given:
Total numbers = 15
Average of all 15 numbers = 20
Average of first 9 numbers = 18
Average of last 7 numbers = 24
Formula:
Sum = Average × Number
Sum of all 15 numbers = 15 × 20 = 300
Sum of first 9 numbers = 9 × 18 = 162
Sum of last 7 numbers = 7 × 24 = 168
Note that the 9th number is included in both sums.
9th number = 162 + 168 − 300
9th number = 30