The average atomic mass is = ∑Mi X (Percentage)i / 100
Mi = atomic mass of isotope
Percentage = the percentage of that isotope
so for given
sotope Atomic Mass Abundance
Z - 24 23.985 amu 78.90%
Z - 25 24.995 amu 10.00%
Z - 26 25.982 amu 11.00%
Z - 28 27.967 amu 0.10%
The average atomic mass
= (23.985 X 78.90% + 24.995 X 10.00% + 25.982X 11.00% + 27.967 X 0.10%) / 100
= 24.3097 amu