Interview Question for Patent Docketing Specialist at Google:

BTW, there is a second solution: Separate out the balls into a group of 4 and a group of 3. Weigh just the group of 4 by dividing them into 2 and 2. Possibility A: If they are evenly weighed, you know the heavier ball is in the group of 3, and you can do the second weight from that group of 3. In that case, the second weighing should be 1 and 1 ball, keeping the third aside. If they are not equal, you will know which is the heavier ball. If they are equal, the ball you kept aside is the heavier one. Possibility B: If the 2 and 2 from the original weighing of 4 balls are not equivalent, you will know which is the heavier group and can weigh those two balls by individually in the second round to determine which is the heaviest ball.