Isotopes Chemistry Question?

Which two of the following are isotopes of the same element" 31/16X, 31/15X, 32/16X... there isn't a divide sign but it's where there is one number on top of another followed by a chemical symbol. And also.. it wants to know. what is the identity of the element whose isotopes you have selected... My answer was the 31/16 and 31/15.. and Ga for the element. but im not sure at all. how do i determine all of this? thanks.