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How many grains of rice would fit on a chessboard if you put one grain on the first square, and double the number for each square on the chessboard?
 Nov 24, 2013
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Your question boils down to a geometric series, and is basically:

1+2+2^1+2^2+....+2^64

the general answer for a geometric series =a(1-r^m)/(1-r); where a=initial term(here 1), r=multiplicative factor(2 here because you are doubling in the next square ), m=number of terms(64, since a chess board has 64 squares)

hence, ans=1*(1-2^64)/(1-2)=2^(64)-1

that is, you need 2^(64)-1 grains of rice, which is a huge number!! It is approximately 2 thousand million billion grains of rice! Assuming you stack the grains of rice width wise, each grain has a width ~ 1/2 mm. This would require a length of 500 billion miles, whereas the Earth's circumference is only 25000 miles.

Hence, you would require 20000 million Earths, with rice stacked along the circumference, to fit your rice!!
 Nov 24, 2013

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