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256 grains of wheat?! Quote: According to legend, Chess was invented by Grand Vizier Sissa Ben Dahir, and given as a gift to King Shirham of India. The king was so delighted that he offered him any reward he requested, provided that it sounded reasonable. The Grand Vizier requested the following:

"Just one grain of wheat on the first square of a chessboard. Then put two on the second square, four on the next, then eight, and continue, doubling the number of grains on each successive square, until every square on the chessboard is reached."

The king thought this a very modest request, promised it, and asked for a bag of wheat to be brought in. However the bag was emptied by the 20th square. The king asked for another bag, but then realized that the entire bag was needed for the next square. In fact, in 20 more squares, he would need as many bags as there were grains of wheat in the first bag!

The number of grains in the last square can be calculated by multiplying 2 times itself 63 times. (This can also be written as 2^63.) You can easily work this out on a pocket calculator. The answer is 9223372036854775808 which is approximately equal to 10^19. (It is actually closer to 0.922 x10^19 = 9.22 x10^18). If you include the grains on the first 63 squares, the sum is about twice as large (but only twice as large! Do you see why? Try it will smaller chessboards, maybe one with only 4 squares. You'll see that you always put more on the last square than on all the preceding squares.)

If all these grains were stacked into a cube, the cube would have 2,642,000 grains on each edge. (If you cube 2,642,000, you get 2 x10^19.) If each grain were 1 mm in size, then each edge would be 2,642,000 mm = 2,642 meters = 2.6 km. The cube would be 2.6 km long, 2.6 km wide, and 2.6 km high. It wouldn't fit on a chessboard.

The amazing feature of this problem is that with just 64 steps, each one quite modest (you are only doubling) you get a huge number. This type of rapid growth is called exponential growth.

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