10 people can share a bucket of coins equally. A monkey steals one coin. The number of coins remaining is therefore 1 less than would allow them to be shared out equally.
One person after another tries to take the coin from the monkey -- but each time, the monkey kills them. Each time a person dies, the number of coins is still 1 less than would allow the remaining people to share them out equally, until we are left with just one person who wisely leaves the monkey alone and takes away ... how many coins?