Friday, 1 July 2011

The last ball

From the Futility Closet:

Posted in Puzzles by Greg Ross on June 30th, 2011
An urn contains 75 white balls and 150 black ones. A pile of black ones is also available.
The following two-step operation occurs repeatedly. First we withdraw two balls at random from the urn, then:
  • If both are black, we put one of them back in the urn and throw other away.
  • If one is black and the other white, we put the white one back and throw the black one away.
  • If both are white, we throw both away and put a black ball from the pile into the urn.
Because the urn loses a ball at each step, eventually it will contain a single ball. What color is that ball?
I will give the answer tomorrow.

I like these kind of riddles, Greg will post his answer tomorrow, meanwhile if you have no clue, see my guess here.

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