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- Cards in the darkness 5 answers
You are blindfolded (and you can’t see a thing). There are $100$ coins lying on a table. Of these coins, $80$ are tails-up and $20$ are heads-up. You need to divide them into two groups of $x$ and $y$ each (with $x+y = 100$). You can pick any coin and flip it (if it is tails, it will become head and if it is head, it will become tail), but you can't see so you don't know which coin you are flipping. In the end, both groups should contain equal number of heads.