If you are given the birth and death dates of two historical figures, you would have no trouble determining if their lives overlapped.

Suppose the birth and death dates for two people are b1 and d1, and b2 and d2. Can you give a statement of the form w>=x and y>=z that will be true if and only if the two lives overlapped?

I found the answer to this to be non-intuitive. The way I tackled it was to find a logical condition for the lives to be non-overlapping, which seemed rather obvious, and then use that to solve the problem.