Looking for a few good C interview problems

Question

I don't want to ask candidates questions; instead, I'd like to present them with a number of challenges to solve. The reason for this is that I've seen people who were great in theory but couldn't handle it when faced with a real-world c# problem.

These c# challenges should be basic enough to answer in 1-20 minutes, but complex enough to weed out those who can't code.

Currently, I ask candidates to reverse a text and eliminate duplicates from a List. This alone eliminates a significant number of persons.

Is there anything else I could use as an example?