I have produced a new kindle ebook containing many of the previous Friday Puzzles! It is called PUZZLED (available in the UK here and USA here) and contains 101 puzzles and the solutions. If you enjoy the Friday Puzzles and the blog, feel free to show your appreciation by buying one of the books.
OK, ad over. Last week’s puzzles was easy. This week’s one is much trickier and was kindly sent to me by Milan V.
Imagine a monastery in which ten of the monks may have a disease which causes them to have blue spots on their foreheads but has no other symptoms. All the monks have taken a vow of silence, they meet just once a day, and there are no mirrors in the monastery, so nobody knows whether he has a blue spot on his forehead or not.
If a monk discovers that he has a blue spot on his forehead, he will have to leave the monastery by the end of the day. All the monks are perfect logicians – that is, they can instantly infer all the logical consequences of any statement made to them – and they all know that all the other monks are perfect logicians.
One day, the Guru, who is known to be truthful, gathers all the monks together and announces “At least one monk in this monastery has a blue spot on his forehead.” Nothing happens for nine days, but on the tenth day, all the monks with blue spots leave.
How many monks left and why?
As ever, please do NOT post your answers, but do say if you have solved it and how long it took! Solution on Monday.