How many moles of magnesium phosphate, Mg3(PO4)2 contains 0.25 mol of oxygen atoms?

Let us suppose that x mol of Mg3(PO4)2 contains 0.25 mol of oxygen atoms. Since each formula unit contains 8 oxygen atoms, x mol of Mg3(PO4)2 contains 8x mol of oxygen atoms.∴8x=0.25⇒x=0.258=0.03125=3.125×10−2