Advanced MathMediumPositive number from a quadratic word problem
Advanced Math practice question
A positive number is such that its square plus the number itself equals 20. What is the number?
Answer: 4
n² + n − 20 = (n + 5)(n − 4) = 0, so n = 4 or n = −5. The positive number is 4.
Common wrong answers
- If you answered 5
- 5 gives 25 + 5 = 30, not 20.
- If you answered 10
- 10 comes from n + n = 20, reading 'square' as a second copy.
- If you answered -5
- −5 is the other root, but the number must be positive.
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