Problem 5 · 2017 Math Kangaroo
Easy
Arithmetic & Operations
sum-constraintcasework
Four of the numbers 1, 3, 4, 5 and 7 are written into the boxes so that the calculation is correct. Which number was not used?
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Answer: C — 4
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Hint 1 of 2
Two sides of the equation must be equal, so you need two pairs from the list that add to the same total.
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Hint 2 of 2
Find two different pairs with equal sums; the fifth number is the one left out.
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Approach: find two pairs with equal sums
- The boxes give an equation of the form (one number)+(one number) = (one number)+(one number), using four of the five numbers.
- Look for two pairs that add to the same value: 1+7 = 8 and 3+5 = 8.
- Those use 1, 3, 5 and 7, so the number left over is 4.
- The unused number is 4.
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