How many pennies are on the chessboard?

If the number of pennies doubles on each square, the 64 squares contain 1, 2, 4, and so on, up to 2^63 pennies. The total is the geometric sum 1 + 2 + 4 + ... + 2^63 = 2^64 − 1. So the correct choice is “2^64 − 1.”

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