OOOOOOMMMMMMMMMGGGGGGGG!!!!! PLEASE HELP ME!?

Sorry for the dramatic question, but I wasn't getting any answers. Anyway, if you are given the names of two isotopes, their percent abundance, and the m of one of them, then how do you find the m of the second one? We will be allowed to reference a periodic table for these types of problems on future tests, but we can't just right the number down;we need to find the m using math. Anyone know how to do this?