Thursday, November 15, 2007

another puzzle

There are N(>1) balls in a row. Each ball is of a different colour. I make lot of swaps where in each swap I interchange the position of two different balls. After K swaps, I see that the original order of balls is restored. In other ways, the order is just the same as it was before we started to do any swaps. Prove that K is always even?

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