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'Knights and Knaves' printed from https://nrich.maths.org/
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.