A newly discovered useless element,X, has two isotopes. The first isotope of X is 12.8% abundant and has an exact atomic mass of 24.01 amu, and the second isotope is 87.2% abundant. If the average atomic mass(in amu) of X is 25.22 amu, what is the exact mass of the second isotope?

Question

jfraser · Answer

you have the percentages of each isotope, and the mass of $one$ of them, as well as the $average$. There's only one missing piece, the mass of the second isotope.
$$24.01amu * (0.128) + (unknown \space amu) * (0.872) = 25.22 amu$$