Well, its solved already, but thought I'd chip in with my view.
There's an intersection set of X and Y, that has the same
number of members. (obviously!).
intersection set, they say, is 20% of X and 30% of Y.
This easily gives us relative sizes of X and Y.
Like if 20 is 20% of X and 30% of Y, we know Y is smaller than X.
Hope that helps.
Who says elephants can't dance?