Now how do we calculate atomic mass when no fractional abundances are given? Well remember adding U, the fractional abundances of all isotopes for an element gives a total of 1 because remember all of them together constitute 100% for that particular element and dividing it by 100 gives us its fractional abundance and that equals one.
Now, this is important for atomic mass questions where there are two isotopes with unknown fractional abundances. So basically, if you're going to have a question where you have to calculate the fractional abundances of all isotopes, it'll only be for two isotopes, not three or more.
