Answer:24.31
Explanation:Contribution made by isotope of mass 23.99= 23.99Ă—78.99=1894.97
Contribution made by isotope of mass 24.99=24.99Ă—10.00=249.9
Contribution made by isotope of mass 25.98=25.98Ă—11.01=286.04
Total contribution=1894.97+249.9+286.04=2430.91
Average mass=2430.91Ă·100
=24.31