Principle of 10 coins
https://gyazo.com/288e7f3b64b1e191202fa0a1e9dd4402
You have 100 coins, 10 of them face down. With your eyes closed, divide them into two groups such that the number of coins facing the other way is the same."
Randomly select 10(s)
x of which are backed.
Of the remaining 90 cards, (10 - x) cards are backsides
So, just flip S over and set the number of coins in S to (10 - x).
When sets of the same size overlap, the sizes of the sets without the overlapped parts also match.
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