# How do you calculate the atomic mass of element X if there are two isotopes of this element (x-24 and x-28) and 75% of all x atoms are x-28 isotopes?

(24 xx 25%) + (28 xx 75%) = 27
75% of all X atoms are X-28, which means that 25% of all X atoms are X-24 as there are only 2 isotopes.