Afictional element has two isotopes and an atomic mass of 87.08 amu. if the first isotope is 86 amu and the second isotope has a mass of 90 amu. which isotope has the greatest natural abundance?

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. 

The most plausible answer is b, duration. the given amount of time is considered how long it last for.

Dmight be the correct answer