Puzzle about sorting coins while blindfolded

1.1k Views Asked by At

You have $n$ coins in front of you, and you are blindfolded. You know that exactly $10$ of the coins are showing heads, the rest are showing tails. How can you sort all the coins into two piles such that the number of coins showing heads are equal in each pile? You are allowed to turn coins, but you have of course no way of distinguishing which side a coin is showing.

1

There are 1 best solutions below

0
On BEST ANSWER

Split the pile into a set of ten on the left and a set of $n-10$ on the right. Flip all coins in the left pile.

Let $h_l$ and $h_r$ denote the number of coins showing heads in the left pile and right pile respectively.

We know that $h_l+h_r=10$ and so $10-h_l = h_r$.

After having flipped the left pile, all the tails there will become heads and all the heads there will become tails. I.e. there will be $10-h_l$ heads in the left pile after flipping.

The two piles will have the same number of heads.