A fool and his money
A fool was sitting under a tree one day when the Devil suddenly materialized in front of him. "Tut, tut," chuckled the Devil. "Laziness is a sin, you know. Walk over to that pond about 100 meters from here and back to the tree... and to give you an incentive, I'll double the amount of money in your wallet every time. But because I'm being so generous, you must give me $16 for for every trip."
"Deal," said the fool. He walked over to the pond and back, took out his wallet and counted the bills... and sure enough, the amount had doubled.
He gave $16 to the Devil and made another walk to the pond and back. His money doubled again, and he gave another $16 to the Devil.
The fool walked to the pond and back a third time. Again the amount in his wallet doubled. But now he had only $16, and had to give it all to the Devil.
"Thanks," said the Devil. He chuckled and vanished as suddenly as he had appeared.
How much, in dollars, did the fool have in his wallet to begin with?
The answer is 14.
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It's easiest to work backwards. Before the third loop, the fool must have had $8 in his wallet, since when it doubled it became $16. Therefore, when the Devil doubled the amount of money after the second loop, the fool had $24 after the doubling, so 24/2 = $12 beforehand.
That means that when the Devil doubled the amount of money after the first loop, it doubled to (12 + 16) = 28. So before the first loop -- hence, at the start -- the fool had (28/2) = $14.