Algebra practice question

Answer: 25

Writing f(x) = mx + b, the condition f(2x) = 2f(x) gives 2mx + b = 2mx + 2b, so b = 0 and f(x) = mx. Then m = 7.5/3 = 2.5 and f(10) = 25.

Common wrong answers

If you answered 15

15 is 7.5 + 7.5, adding rather than scaling by 10/3.

If you answered 75

75 is 7.5 × 10, multiplying by x instead of by x/3.

If you answered 22.5

22.5 is f(9).

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