Consider a hypothetical element x that consists of two isotopes, one with a mass of 41.99998 amu and the other with a mass of 44.99987 amu. their percent abundances are 33.3333% and 66.6667%, respectively. without doing an actual calculation, the mass closest to the atomic mass of x is

To determine the average atomic weight of the hypothetical element X, you essentially take the average of the isotopes based on the percent abundance of each isotope present.

However, to do this "without doing an actual calculation" is not possible. But we can do it with very simplified calculations. We have one isoptope of X that is 42 amu that is 1/3 of the element, and the other isotope which is 45 amu that is 2/3 of the element.

A simple calculation to find the mass of X is as follows:

(1/3)(42 amu) + (2/3)(45 amu) = atomic weight of X.
14 + 30 = atomic weight of X
Atomic weight of X = 44 amu.

The mass closest to the atomic mass of X is 44 amu.