Using your Head is Permitted

July 2011 riddle

A group of n people are hiking through a forest. With them is a monkey. Just when they are about to stop and camp for the night, they find a large pile of bananas. They agree to divide the bananas equally and to eat them for breakfast, but postpone actually doing anything with this decision until the morning.

In the middle of the night, one of the party wakes up hungry. He decides not to wait for morning and to eat his share immediately. He divides the bananas into n equal batches, but in order for the equality to be perfect he gives the monkey one banana. He then eats his share and puts all n-1 remaining batches back in one pile.

After he falls back to sleep, another of the party wakes up. He, too, decides to eat his share immediately, and goes through exactly the same process as the first, including the giving of one banana to the monkey for the numbers to divide properly.

After the second man falls asleep, a third wakes up and repeats the same process. Eventually, each person in the party wakes up once during the night, gives the monkey a single banana and eats 1/n of the remaining bananas. Altogether, the monkey eats n bananas during the night.

Come morning, none of the people in the party remember the events of the previous night. They divide the bananas evenly, each getting 1/n of the remaining bananas and give one banana to the monkey for the numbers to divide properly.

The question: what, as a function of n, are all the possibilities for the number of bananas in the original bunch? Prove your answer.

List of solvers:

Christian Blatter (1 July 19:56)
Dan Dima (1 July 20:12)
Lukasz Bolikowski (2 July 02:59)
Joseph DeVincentis (2 July 06:04)
Gaoyuan Chen (2 July 17:55)
Daniel Bitin (2 July 21:55)
Sylvain Becker (3 July 02:45)
Jan Fricke (3 July 02:50)
Oded Margalit (3 July 08:06)
Yan Wang (3 July 23:52)
Djinn Lu (5 July 01:43)
Naftali Peles (5 July 08:55)
Lu Wang (5 July 12:16)
Albert Stadler (5 July 15:55)
Harsha HS (6 July 19:42)
Liubing Yu (14 July 11:44)
Itsik Horovitz (31 July 23:14)

Elegant and original solutions can be submitted to the puzzlemaster at Names of solvers will be posted on this page. Notify if you don't want your name to be mentioned.

The solution will be published at the end of the month.


To solution

Back to main page