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AlgebraEasyTwo lengths from their sum and difference

Algebra practice question

Two planks have a combined length of 13 feet, and one is 3 feet longer than the other. How long, in feet, is the longer plank?

Answer: 8

l + s = 13 and l − s = 3 give l = 8.

Common wrong answers

If you answered 5
5 is the shorter plank.
If you answered 10
10 adds 3 to 13 and halves wrongly; (13 + 3)/2 = 8.
If you answered 6.5
6.5 halves 13.

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