How do I "calculate" the "relative abundance" of isotopes of an Element (See question details inside)?

The element Europium exists in nature as two isotopes: Eu-151 has a m of 150.9196 amu, and Eu-153 has a m of 152.9209 amu. The average atomic m of Europium is 151.96 amu. How do I go about calculating the "relative abundance" of the two isotopes? What are the steps involved here?