I can't figure this out, it seems almost like a trick question.

When is |A u B| = |A| + |B|?

I'm not even sure what type of answer my professor is looking for so I've been working with quantifiers.

Using quantifiers the closest to the correct answer I can come up with is something like this:

|A u B| = |A| + |B| when "for all"x(x is an element of A and x is an element of B), but it still doesn't satisfy the problem

Thank you