Puzzle No.3/Solution

Efficient Weighing

He divides the bricks into 3 groups of 3, 3, and 2 bricks. Then he weighs the 2 sets of 3 bricks against each other. If they balance, then the heavier brick is in the group containing 2 bricks and will be determined by a second weighing.

If they do not balance, then he weighs 2 of the group that was heavier on the first weighing. If these 2 balance, then the third one is the heavier brick. If they do not balance, then the scales tell which is the heavier brick.