Select any 10 coins from the 100 and flip all the 10. Now you will have two groups: one with 90 coins and the other with 10 coins.
Suppose that out of the 10 selected coins, 4 had heads. This means that out of the ramaining 90, 6 have heads. Now after flipping, the other group will also have 10 - 6 = 4 number of heads!
You can see for yourself that this works for all cases.

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