1.5 mole of ${\mathrm{O}}_2$ combines with Mg to form oxide MgO. The mass of Mg (at. mass 24) 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 mole of oxygen react with 1 mole of Mg

1.5 mole of oxygen react with $\frac{1.5}{0.5}=3\mathrm{mole}$s of Mg

                                                                          =$24\times 3=72\mathrm{g}.$ of Mg