Question: element x has two isotopes?
Answer: Element X has two isotopes. If 72.0% of the element has an isotopic mass of 84.9 amu and 28.0% has an isotopic mass of 87.0 amu, the average atomic mass of element X is numerically equal to (1) (72.0+98.9)(28.0+87.0), (2) (72.0-84.9)(28.0-87.0), (3) (0.720)(84.9)+(0.280)(87.0), (4) (72.0)(84.9)+(28.0)(87.0).