Advanced Math practice question

Answer: 7

Let the number be x: x(x − 4) = 21, so x² − 4x − 21 = 0, which factors as (x − 7)(x + 3) = 0. The positive solution is x = 7. Check: 7 × 3 = 21.

Common wrong answers

If you answered 3

3 is the SMALLER of the two factors (7 − 4), not the number itself.

If you answered -3

−3 is the other solution of the quadratic, but the question asks for the positive number.

If you answered 5.25

5.25 is 21/4, dividing the product by 4 — but 4 is subtracted, not multiplied.

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