Mole Concept Questions

If we consider that $1/6$ in place of $1/12$ mass of carbon atom is taken to be the relative atomic mass unit, the mass of one mole of a substance will:

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