Nitrogen has an average atomic mass of 14.00674. it has two isotopes, n-14 and n-15, with atomic masses of 14.0031 amu and 15.001 amu, respectively. what is the abundance of n-15?

In this case, we are going to assume that there are 100 atoms to make things easier. 

Let R% be the abundance of n-15. With this in mind, we calculate the abundance of n-14 to be 100%-R%

14.0031*(100-R)% + 15.001 * R%= 14.00674 
In this case, we can delete or ignore the % sign since we do not want to carry it around, however, we need to keep in mind that the final answer is in %

14.0031*(100-R) + 15.001 * R= 14.00674 
1400.31-14.0031R+15.001R=1400.674
0.9979R=0.364
R=0.3648

Then, the abundance of n-15 is 0.3648%