You are given 8 identical looking balls. One of them is heavier than the rest of the 7 (all the others weigh exactly the same). You are provided with a simple mechanical balance scale. How can you find the heavier ball by using the scale only twice?
Answer
Take 6 balls and split them into 2 groups of 3 balls each. Use the scale to weigh the 2 groups.
If one of the groups is heavier than the other group, pick 2 balls from the heavier group and use the scale to weigh them. If they are equal in weight, the other ball in that group is the heavy one.
If both the groups are equal in weight, then pick the remaining 2 balls and use the scale to determine the heavier ball.