Can someone help me and explain why they got what they got?

24.32 amu

Explanation:

From the question given above, the following data were obtained:

Isotope A (Mg–24):

Mass of A = 24 amu

Abundance (A%) = 79%

Isotope B (Mg–25):

Mass of B = 25 amu

Abundance (B%) = 10%

Isotope C (Mg–26):

Mass of C = 26 amu

Abundance (C%) = 11%

Average atomic mass of Mg =?

Average atomic mass = [(Mass of A × A%)/100] + [(Mass of B × B%)/100] + [(Mass of C × C%)/100]

= [(24 × 79)/100] + [(25 × 10)/100] + [(26 × 11)/100]

= 18.96 + 2.5 + 2.86

= 24.32 amu

Thus, the average atomic mass of Mg is 24.32 amu

the answer is d. 74.02