The atomic mass of a certain element is summation of the product of the decimal equivalent of the percentage abundance and the given atomic mass of each of the isotope. If we let x be the percentage abundance of the 86 amu-isotope then, the second one is 1-x such that,
x(86) + (1 - x)(90) = 87.08
The value of x from the equation is 0.73. This value is already greater than 0.5. Thus, the isotope with greatest abundance is that which is 86 amu.
