There are 100 coins scattered in a dark room. 90 have heads facing up and 10 are facing tails up. You cannot tell which coins are which.
How do you sort the coins into two piles that contain the same number of tails up coins?


Another Dark Room
How do you sort the coins into two piles that contain the same number of tails up coins? 4 guesses to Another Dark Room 

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You split into 90 and 10, and the group of 10 you turnover. All the heads that are in the second group of 10 at first will now become tails, and this is exactly how many tails there are in the group of 90. t1 + t2 = 10 and h2 + t2 = 10, so h2 = t2 ! Turning these unknown number of h2s into t’s make it the same number of t’s as t1!
Quorum almost has it, but not quite. You split the coins into two groups of 50, THEN you turn over all the coins in one of the groups.
*Kirumo* Damn you, autocorrect!
Kirumo, you are today’s winner!
The piles don’t need to be the same size, so make a pile of 10 coins and a pile of 90 coins, flip all of the ten coins and it is guaranteed that the piles have the same number of tails.