Find the geometric mean of 4 and 16.

Explanation

• For two numbers the geometric mean is √(a × b), so it is √(4 × 16) = √64 = 8.
• The geometric mean multiplies the numbers, while the arithmetic mean of 4 and 16 would be (4 + 16) ÷ 2 = 10.
• Option C 10 is that arithmetic mean, and it is the most common slip in this question.
• Useful check: 4, 8 and 16 form a geometric progression with common ratio 2, so 8 sits correctly in the middle.
• The geometric mean is never greater than the arithmetic mean, so an answer below 10 is expected here.