WAEC 2018 Mathematics · Question 21

The mean of 1, 3, 5, 7 and x is 4. Find the value of x

Options

Answer: B. 4

Explanation written with AI and checked against the answer key

Analyze the question: We are given five numbers (1, 3, 5, 7 and x) whose mean is 4. We need to find x. Concept: Mean = (sum of all values) ÷ (number of values) Solution: Sum of the known numbers: 1 + 3 + 5 + 7 = 16 Number of values = 5 Set up the equation: (16 + x) / 5 = 4 Multiply both sides by 5: 16 + x = 20 x = 20 − 16 = 4 Check: (1 + 3 + 5 + 7 + 4) / 5 = 20 / 5 = 4 ✓ Answer: x = 4, which is option B. Tip: When a question gives the mean, always find the total first by multiplying mean × number of values (here 4 × 5 = 20). Then subtract the sum of the known values (20 − 16) to get the missing one. It's quick and avoids mistakes.

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