The average atomic mass of Boron is 10.8. It has two isotopes of masses 10 and 11 respectively. What is the percentage of isotope with the average mass of 10?

Explanation

The formula for calculating the average atomic mass is[ (Mass of one isotope x percentage abundance) + (mass of other isotope x percentage abundance) divided by 100]. In this question, aside from using the formula for each option which would be a bit time consuming, you can take clues from the question itself. The average mass of Boron is 10.8, which is closer to 11 than 10. As such, you can conclude that the abundance of 10 isotope is lesser than the abundance of the 11 isotope, as such, its abundance MUST be less than 50%. The only option with an abundance less than 50 percent is option A, 20 percent. If you check this with the formula, the answer comes out to be correct:[(10 x 20) + (11 x 80) divided by 100] is 10.8.