Answer
24.3051 amu
Work Step by Step
To find the average atomic mass we need to find the weighted average of the three isotopes. We do so by taking the sum of mass of each isotope times its abundance as follows:
23.98504amu * 0.7899 + 24.98584amu*0.1000 + 25.98259amu * 0.1101 = 24.3051 amu