Ahypothetical element consists of 3 isotopes, and the atomic weight is 39.2 amu. isotope1 is 78.8% with mass number of 39; isotope 2 is 3.1% with mass number of 41. what is the mass of isotope 3?
1. 38
2. 43
3. 40
4. 42

The atomic mass of the elements is the weighted average of the masses of the isotopes.
 
This is, atomic mass = % of isotope 1 * mass number of isotope 1 +  % of isotope 2 * mass number of isotope 2 + +  % of isotpe 3 * mass number of isotope n.

Applied to our hypotethical element with 3 isotopes:

aomic mass of the element = 39.2 = 78.8% * 39 + 3.1% * 41 + 18.1% * x

Solve for x, x = [39.2 - 30.732 - 1.271] / .181 = 39.76 = 40

40

It would be