Atomic Abundance Example Chemistry Problem

The element boron consists of two isotopes, ¹⁰₅B and ¹¹₅B. Their masses, based on the carbon scale, are 10.01 and 11.01, respectively. The abundance of ¹⁰₅B is 20.0%.
What is the atomic abundance of and the abundance of ¹¹₅B?
Solution
The percentages of multiple isotopes must add up to 100%.
Since boron only has two isotopes, the abundance of one must be 100.0 - the abundance of the other.
abundance of ¹¹₅B = 100.0 - abundance of ¹⁰₅B
abundance of ¹¹₅B = 100.0 - 20.0
abundance of ¹¹₅B = 80.0
Answer
The atomic abundance of ¹¹₅B is 80%