Brain Teasers/Split 100 coins with 10 heads up evenly

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You have a 100 coins laying flat on a table, each with a head side and a tail side. 10 of them are heads up, 90 are tails up. You can't feel, see or in any other way find out which side is up. Split the coins into two piles such that there are the same number of heads in each pile.

Answer

  • Divide the coins randomly into two piles, one with 90 coins and the other with 10 coins.
  • Let's say the pile with 90 coins has n (0 to 10) coins with head up. Then the other pile has 10 - n coins with head up.
  • Flip all the coins in the 10 coin pile. Now the pile has n coins with head up.
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