There are two stable isotopes of bromine . their masses are 78.9183 amu and 80.9163 amu. if the average atomic mass of bromine is 79.90 amu, what is the natural abundance of the lighter isotope?

There is more of the lighter isotope, because the average is close to the lighter isotope. When doing the calculation for the average atomic mass, it considers the fraction of the isotopes available in a sample. Therefore samples of the element that have a values that do not deviate far from the sample are more abundant.