1.5 moles of O₂ combine with Mg to form oxide MgO . The mass of Mg (At. mass24) that has combined is

$\underset{1 \mathrm{mole}}{\mathrm{Mg}}+\underset{0.5 \mathrm{mole}}{\frac{1}{2}{\mathrm{O}}_2}\to \mathrm{MgO}$
0.5 moles of oxygen react with 1 mole of Mg
1.5 moles of oxygen react with $\frac{1.5}{0.5} = 3 \mathrm{moles}$
24 x 3 = 72 g