One out of 9 otherwise identical balls is overweight. How can it be identified after 2 weighings?
Weigh a group of 3 balls against another group of 3, then you'll know which group of 3 contains the heavy ball. Pick 2 balls from that group and weigh one against the other.
A little boy goes shopping and purchases 12 tomatoes. On the way home, all but 9 get mushed and ruined. How many tomatoes are left in a good condition?