A puzzle from The Book of the Thousand Nights and One Night:
“Solve the following problem if you can: a flock of pigeons alighted upon a tree, some perching upon the upper branches and some upon the lower; those upon the upper branches said to those upon the lower: ‘If one of you flies up to us our number will be double yours; if one of us flies down to you, our numbers will be equal.'”
How large was the flock?
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“There were twelve pigeons in all, seven upon the upper branches and five upon the lower branches. If one of the lower ones joined the higher, there would be eight above, which would be the double of four; but if one of the seven flew down to the lower ones, there would be six in each position. But Allah is all-knowing.”
Via Aplusclick‘s excellent collection of math and logic problems. (Thanks, Igor.)
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