Thursday, August 24, 2006

Brainteaser - Satanic monks

Reader JM_Brazil has kindly sent in this great brainteaser:
  • There's a monastery that is inhabited by Monks.
  • They are sworn not to communicate with each other in any way.
  • They have no mirrors, and no other means by which a monk can see his own face.
  • They see each other only once each day when they all gather together for afternoon prayers.
  • One day the head monk shows up to inform them that at least one of them is possessed by Satan. (The head monk can talk, obviously.)
  • They are informed that if one is possessed it will be indicated with an X on the forehead.
  • A possessed monk must leave the monastery.
  • Day 1 goes by, no one leaves. Day 2 arrives, no one has left. Day 3, all have left.

Question: How many monks inhabited the monastery?

(Apparently the possession occurred after a visit from the Monk pictured.)

Update: the solution