You are given 18 silver coins but you find out that one of the coins (you don't know which one) is fake and weighs less than any of the others. How can you detect the fake coin using a normal balance scale only three times?

You divide the coins into three piles of six, pile A, pile B, and pile C. You put pile A on one side ofthe balance and pile B on the other side. Whichever is lighter will contain the fake coin. If they weigh the same, the coin will be in pile C.

You now divide the coins into three piles of two (A, B and C again) and weigh pile A against pile B. Once again you will be able to identify which pile contains the fake.

Now you weigh one against the other - the lighter coin is the fake.