You're blindfolded in front of 20 coins. 10 are heads up. You can flip coins but not see or feel them. How do you make two piles with the same number of heads?
Hint: The piles don't need to be the same size.
Take any 10 coins into a pile and flip all of them.
If your 10-coin pile has h heads, the other pile has 10 − h heads. Flipping your pile gives it 10 − h heads too, so both match.
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