What is fractional abundance?

i need help with my chem homework, i can't find the defination if fractional abundance and i need herlp on this problem 13. Problem on fractional (relative) abundance: An element is found to be composed of two isotopes. Isotope A has a fractional abundance of .80 and a m of 10.80 amu. Isotope B has a fractional abundance of .20 with a m of 9.50 amu. What is the atomic weight of this element?