There are two stable isotopes of gallium; there masses are 68.9257 amu and 70.9249 amu. If the average atomic mass of gallium is 69.73 amu, what is

There are two stable isotopes of gallium; there masses are 68.9257 amu and 70.9249 amu. If the average atomic mass of gallium is 69.73 amu, what is the natural abundance of the lighter isotope?