B) Calculate the relative atomic mass for magnesium using the information below (to 3 significant figures).
Magnesium has three common isotopes, magnesium-24, magnesium-25 and magnesium-26.
Their abundances are 79.0%, 10.0% and 11.0%, respectively.

Relative atomic mass  = 24.32 amu.

Explanation:

Given data:

Relative atomic mass of Mg = ?

Abundance of Mg-24 = 79.0%

Abundance of Mg-25 = 10.0%

Abundance of Mg-26 = 11.0%

Solution:

Relative atomic mass = (abundance of 1st isotope × its atomic mass) +(abundance of 2nd isotope × its atomic mass) +  (abundance of 3rd isotope × its atomic mass)  / 100

Relative atomic mass  = (79.0×24)+(10.0×25) +(11.0×26) /100

Relative atomic mass =  1896 + 250 + 286 / 100

Relative atomic mass  = 2432 / 100

Relative atomic mass  = 24.32 amu.

the answer is b & c i believe : )