So when you are calculating relative abundance you are actually calculating the atomic mass on the periodic table. So you are given the two isotopes of Carbon C-12 and C-13. Now you know that the atomic mass of C is 12.01 so you have to see that you have a lot more C-12 because the atomic mass is close to 12. Now you can calculate them out like this (0.99 x 12.0) +(.01 x 13.00)=12.01.
