There is a stack of 100 coins. Each coin has a silver side and a golden side. 20 coins are silver side up. The rest is golden side up. You are in a totally dark room. How do you make 2 stacks that contain as many coins with silver side up ? (we don’t care about how many are golden side up in each group) There is no way to differentiate the coins in the dark (by touch or other means.)
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You take 20 coins from the stack and flip them to create another stack. The two stacks are the original stack withtout the 20 coins, and the stack of 20 coins you created.
If the 20 coins you select are golden side up, then you will flip them and have 20 silver coins in each stack.
If the 20 coins you select are silver side up, then you will flip them and have 0 silver coins in each stack.
If the 20 coins you select contain n silver and 20-n gold coins, then you will flip them and have 20-n silver coins in each stack.