If the average atomic mass of boron is , what is the percent abundance of boron-11 （mass of ） if the only other isotope is boron-10 （mass of ）? Given the formula for average atomic mass:

Therefore in summary, we are given the following known quantities

With the following unknown quantities

And asked to find the percent abundance of boron-11 （）. Therefore our equation is

We have two unknowns, but luckily we can use the following fact/relation and therefore express the percent abundance of boron-10 in terms of the percent abundance of boron-11

Therefore

But we aren't done, we have to compute significant figures.

Therefore, we round our answer to 2 sig figs, yielding