Pot puzzle
An old woman who did not trust banks converted all her wealth into a large number of gold coins, storing them in pots. She then buried all the pots in a house, some in each room.  Curiously, each room had as many pots as the number of coins in each pot, and the house had exactly as many rooms as the number of pots in each room.
When she died, the woman left a will leaving all the gold coins to her three grandchildren, except for one pot to be given to her faithful housekeeper. Since her grandchildren always fought each other, the woman made a conditional will: all information about the treasure (location, how many pots etc) would be revealed to them only if they promise that they would divide their legacy into exactly equal shares. Otherwise they get nothing.
The grandchildren are worried: without knowing how many coins there were, how could they promise to divide the legacy equally?