There is a group of 20 people in a room. Each of them is given an envelope with a random number from 0 to 19, written inside the envelope. The numbers don’t need to be distinct, i.e. two people may get the same number.
Nobody knows what number they got. Then, everybody shows their number to everyone else, but without looking at it - so each person can see what number other people got, but not their own. Each person is asked to guess what number is in their envelope.
What strategy can the people agree on to ensure that at least one person guesses their number correctly?
Puzzle made by
Senior AI specialist
Jakub has been working on combinatorial optimization problems in the Visma Resolve team for the last 3 years. He enjoys film, music and writing about himself in the third person.