If you haven't done any of the other coin problems, then you might want to go back and try those now, this one is very difficult, even if you have figured out the other ones.

This time, as the title implies, there are 39 coins, and one is fake. You have a balance scale, which can be used 4 times.

How would you find the fake coin?

divide the coins into piles of 15, leaving 1 out. weigh the piles against each other; if one is heavier, it contains the odd coin. if they weigh the same, the coin you left out is the odd one. if not, take the heavier pile and divide it into piles of 7, leaving 1 out again. repeat the process another 2 times and you will get your odd coin.

Posted by me on 2003-06-05 23:11:41