Pierre said, "Just one of us is telling the truth".
Qadr said, "What Pierre says is not true".
Ratna said, "What Qadr says is not true".
Sven said, "What Ratna says is not true".
Tanya said, "What Sven says is not ture".
How many of them were telling the truth?
If you liked this problem, here is an NRICH task
that challenges you to use similar mathematical ideas.
This problem is taken from the UKMT Mathematical Challenges.