On a mission to a newly discovered planet, an astronaut finds gallium abundances of 60.11 % for 69ga and 39.89 % for 71ga. what is the atomic mass of gallium for this location? the mass of 69ga is 68.7200 amu . the mass of 71ga is 70.9200 amu .

The average atomic mass of the substance can be calculated using the following rule:
average atomic mass = (mass of 1st isotope x its percentage of abundance) 
                                      + (mass of 2nd isotope x its percentage of abundance)

Now, lets check the given:
mass of 1st isotope = 68.72 grams
percentage of its abundance = 60.11%
mass of 2nd isotope = 70.92 grams
percentage of its abundance = 39.89%

So, we just need to substitute with the givens in the above mentioned equation to get the average atomic mass as follows:
average atomic mass = (68.72 x 0.6011) + (70.92 x 0.3989) = 69.59758 g