Chemistry Exam Question? (Isotope Mass Question - GCSE)?

This is an exact copy of the question from a practice paper, but I cannot get my head around it! Q: Element S exists as two isotopes, mass number 10 and 11. A sample of S contains 20% of the isotope with mass number 10 and 80% of the isotope with mass number 11. What is the relative atomic mass of element... show more This is an exact copy of the question from a practice paper, but I cannot get my head around it!

Q:
Element S exists as two isotopes, mass number 10 and 11.
A sample of S contains 20% of the isotope with mass number 10 and 80% of the isotope with mass number 11.

What is the relative atomic mass of element S?

Is it
A) 10.2
B) 10.5
C) 10.8
D) 21


An explanation of how you work it out would be great! Thanks!
3 answers 3