There are plenty of problems to choose from, and they all require you to puzzle through some sort of tricky question. I like how the questions are worded, and include an example answer.
I spent 20 minutes working through this one:
2520 is the smallest number that can be divided by each of the numbers from 1 to 10 without any remainder.
What is the smallest number that is evenly divisible by all of the numbers from 1 to 20?
Like wind sprints, it left me feeling out of shape and in need of more exercise.