Element X has two known isotopes. If 83% of the sample weighs 76 amu and the remaining 17% weighs 73.6 amu, what is the atomic mass of element X

The atomic mass of an element is the average mass of the atoms of an element measured in atomic mass unit (amu, also known as daltons, D). The atomic mass is a weighted average of all of the isotopes of that element, in which the mass of each isotope is multiplied by the abundance of that particular isotope. (Atomic mass is also referred to as atomic weight, but the term "mass" is more accurate.)

it is the most commonly accepted one