Can somebody please answer this question

Question

Element A has two isotopes. The first isotope is present 12.56% of the time and has a mass of 231.06. The second isotope has a mass of 207.39. Calculate the atomic mass of element A. 

Answer 1

I like the explanation provided by @Michael L, but thought I'd post a convenient way to solve this as well.

We are looking basically at a weighted average of the isotopes to find the average atomic mass.

(12.56%)(231.06) = 29.02

(87.44%)(207.39) = 181.34

Add these together

29.02 + 181.34 = 210.36

Answer 2

The atomic mass would be 210.36.

Explanation: If the first isotope is there 12.56% of the time, the other isotope must be there 100-12.56=87.44% of the time. Let's say you scooped up 10000 atoms of Element A. Then 1,256 of the atoms would have mass 231.06 while the other 8,744 would have mass 207.39. The total mass would be 1,256*231.06 + 8744*207.39 = 2,103,630. The average mass, which is what we want, is that divided by the 10,000 we scooped up, or 210.363.