How many moles of magnesium phosphate, ${\text{Mg}}_3({\text{PO}}_4{)}_2$ will contain $0.25\text{mole}$ of oxygen atoms?

One molecule of ${\text{Mg}}_3({\text{PO}}_4{)}_2$ contains $2\times 4=8$ oxygen atoms.Therefore, $1\text{mole}$ of ${\text{Mg}}_3({\text{PO}}_4{)}_2$ contains $8\text{moles}$ of oxygen atoms.Let $n$ be the moles of magnesium phosphate.
$$8n=0.25$$
$$n=\frac{0.25}{8}=\frac{25}{800}$$
$$n=0.03125=3.125\times 10^{-2}\text{moles}$$