Algebra practice question

Answer: 7

The slope is (16 − 7)/(4 − 1) = 3, so f(x) = 3x + b. From f(1) = 7: 3 + b = 7, b = 4. Then 3x + 4 = 25 gives 3x = 21, x = 7.

Common wrong answers

If you answered 9

9 is 25 − 16, the gap in outputs, not the input. Divide that gap by the slope (9/3 = 3) and add it to x = 4: x = 7.

If you answered 13

13 is f(3), or 25 − 12 — a value of the function, not the input that produces 25. Solve 3x + 4 = 25.

If you answered 31

31 is f(9). Check: the question asks which x gives 25, and 3(7) + 4 = 25.

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