Magnesium Isotopes

Question

If magnesium has two isotopes, one that weighs 24 amu and another that weighs 25, and one out of every three atoms of magnesium has a mass of 25, then what is the atomic mass for naturally occurring magnesium?

Answer 1

If one out if three is 25 amu, then that equates to 33.33% or o.333. That means the other isotope is 0.667 fractional abundance. 

(0.333)(25) + (0.667)(24) = 8.325 + 16.008 = 24.333