Chessboard and Rice Quiz Puzzle Answer

This week's puzzle is from the mists of antiquity. There are many variations on the story behind the puzzle, but the puzzle itself remains the same in all of them. We hope you enjoy.

The story goes that a King or other ruler wants to reward one of his subjects and asks what they would like, expecting the request to be for money, gold or something in that vein.

Instead the request is for one grain of rice to be placed a the first square of a chessboard, two grains of rice on the next square, four grains on the next square, and so on, doubling each time.

The King thinks this is a very meagre reward and urges the subject to reconsider, but he is unmoved - this is the reward he wants.

How many grains of rice does he receive?

The number of grains of rice increases very rapidly, with the 64th and final square containing 2 ^ 63 grains of rice. The total number of grains of rice is 2 ^ 64 -1 or 18,446,744,073,709,551,615 which works out at about 1.4 trillion metric tons; more rice than there is on the planet.