ditto all that.
Here's my "let's learn something from this" comment:
Whenever the GMAT asks for the value of several variables together, rather than a particular variable, it's likely that you'll find a way to solve for the group without having to solve for them individually.
In this example, they ask for 3x + 2y, not x and y individually. You can get their individual values if you want, you'd still get 20, but probably you don't have to. Notice that when we get the answer, we still have no idea what x and y are. That doesn't matter at all.
So this is the "red flag". Many variables in a single question usually points to a trick somehow.