Five sailors, a monkey, and a pile of coconuts nobody can agree how to split
Five sailors gather a pile of coconuts, then sleep planning to divide them fairly at dawn. In the night, each sailor in turn wakes, divides the pile into 5 equal piles with exactly one coconut left over (given to the monkey), hides his own fifth, and reassembles the rest, unaware the others did the same. At dawn the remaining pile again splits evenly into 5, with one left for the monkey. What's the fewest coconuts they could have started with?
Reveal the answer
3,121 coconuts. A 1926 magazine puzzle by Ben Ames Williams that deliberately withheld its answer drew over 2,000 reader letters demanding a solution; Martin Gardner later popularized an elegant trick for solving it involving negative coconuts.
— Ben Ames Williams, Coconuts — The Saturday Evening Post, 9 October 1926