Peter Winkler's excellent book "Mathematical Puzzles: A Connoisseur's Collection" states this easy problem is known to frustrate even great mathematicians. Can you figure it out?

Cut The Knot
http://www.cut-the-knot.org/proofs/chocolad.shtml

I was also emailed the problem, which was given at a Greek university to first year students. I suggested to submit to Puzzling StackExchange where it got a great response:
https://puzzling.stackexchange.....com/questions/72994

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