The average of 7 numbers is 81. If the average of the first 3 is 87 and last 3 is 78, what is the 4th number?
Show answer & explanation
Correct answer: B
Explanation
Given:
Average of 7 numbers = 81
Average of the first 3 numbers = 87
Average of the last 3 numbers = 78
According to the formula:
Average = (Sum of numbers) / (Number of numbers)
Total sum of the 7 numbers = 81 × 7 = 567
Sum of the first 3 numbers = 87 × 3 = 261
Sum of the last 3 numbers = 78 × 3 = 234
Fourth number = 567 − (261 + 234) = 567 − 495 = 72