You have 27 coins...
Divide them into two groups of 13, holding one coin out.
the measurements on the scale will read 130g for one group and either 130, 131, or 129 for the second. Disregard the group with the weight of 130g.
Assuming you didn't pull the odd coin, again split the coins in the lighter/heavier group in half once again leaving out one coin
This time the scales would read 60g for one group and 60/61/59 for the other
The third time, you'll have no need to remove a coin, so just weigh the two groups of three.
Same as before the scales will read 30g for one and either 30/31/29 for the third.
You're now down to three coins. Hold one out and weigh the final two, and voila!
Edited by Shachi, 23 September 2009 - 01:13 AM.