Puzzle

I sent this puzzle out to the team last week and had some great responses.

Thanks to my friend Mark for posing the puzzle to me in the first place.

You have 100 discs on a tray. The discs are white on one side and black on the other. That is: you can flip a disc in order to switch its colour. At the start of the task, 90 of the discs are black and 10 are white. You are now blindfolded.

Your task is to separate the discs into two piles such that each pile has the same number of white discs in it.