Problem 4 · 2023 Math Kangaroo
Easy
Arithmetic & Operations
work-backward
John throws 150 coins onto a table. 60 of them show “heads”, the others show “tails”. He wants the same number of coins to show heads as tails. How many coins that show heads does he have to turn over?
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Answer: B — 15
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Hint 1 of 2
First work out how many coins currently show tails.
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Hint 2 of 2
Each coin you flip changes the head-count by one; aim for an equal split of 75 and 75.
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Approach: balance the two counts with single flips
- There are 150 coins: 60 heads and 150 − 60 = 90 tails.
- For an even split he needs 75 heads and 75 tails.
- He must move 15 coins from the larger pile to the smaller, i.e. flip 15 coins.
- So the answer is 15 (B).
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