How many moles of magnesium phosphate, Mg₃(PO₄)₂ will contain 0.25 mole of oxygen atoms ?

$$\begin{gathered} 1 \mathrm{mole} \mathrm{of} {\mathrm{Mg}}_3{\left({\mathrm{PO}}_4\right)}_2 \mathrm{contains} 8 \mathrm{moles} \mathrm{of} \mathrm{oxygen} \\ ????? -----------0.25 \mathrm{moles} \mathrm{of} \mathrm{oxygen} \end{gathered}$$

$\Rightarrow \frac{0.25\times 1}{8}=0.03125 =3.125\times {10}^{-2}$