An element found in another galaxy exists as two isotopes. if 80.0 percent of the atoms have an atomic mass of 80.00 atomic mass units and the other 20.0 percent have an atomic mass of 82.00 atomic mass units, what is the atomic mass of the element?

Relative atomic mass of the element is the weighted average atomic mass of the element with regard to the abundance of the isotopes in nature.
atomic mass of the element is the sum of the products of the isotope mass by the percentage abundance of that isotope 
isotope 1 - abundance 80 % - 80.00 a.m.u
isotope 2 - abundance 20 % - 82.00 a.m.u 
atomic mass = 80.00 x 80% + 82.00 x 20% 
                     = 64.00 + 16.40 = 80.40 a.m.u
atomic mass of the element is 80.40 a.m.u

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