1) Divide the coins into three piles as equally as possible

2) Measure two of the piles that have

the same number of coins (i.e., if all three have the same number, any two. If there is one left over, measure the two that don't get it. If there are two left over, measure the two that get them.)

3) If the two you measured are

identical, the light coin is out in the

non-measured group. Otherwise, it's in the

lighter group.

4) Repeat the previous steps with the

offending pile

Should be faster.

Don't know how to prove it's

the fastest, though.