WAEC 2022 Mathematics · Question 45

The mean of a set of 10 numbers is 56. If the mean of the first nine numbers is 55, find the 10th number.

Options

A.75
B.65
C.55
D.45

Answer: B. 65

Explanation written with AI and checked against the answer key

Analyze the question: We are given the mean of all 10 numbers and the mean of the first 9 numbers, and we need to find the 10th number.

Concept: Mean = (Sum of numbers) ÷ (Number of numbers), so Sum = Mean × Number of numbers.

Solution:

Sum of all 10 numbers = 56 × 10 = 560
Sum of the first 9 numbers = 55 × 9 = 495
10th number = 560 − 495 = 65

Check: (495 + 65) ÷ 10 = 560 ÷ 10 = 56 ✓

Tip: In "missing number" mean problems, always convert each mean back into a total (mean × count). Then subtract the smaller total from the larger one to get the missing value.

AI can make mistakes. Report anything that looks wrong.