A magical island is inhabited entirely by knights (who always tell the truth) and knaves (who always tell lies).
One day 25 of the islanders were standing in a queue.
The first person in the queue said that everybody behind was a knave.
Each of the others in the queue said that the person immediately in front of them in the queue was a knave.
How many knights were there in the queue?
If you liked this problem, here is an NRICH task
which challenges you to use similar mathematical ideas.