Essential Chemistry for IGCSE - Chemical Calculations (Section 2 - No. 34)

In the magnesium oxide example, how is the mass of oxygen calculated?

Mass of magnesium - mass of oxygen

Mass of magnesium oxide - mass of magnesium

Mass of oxygen + mass of magnesium

Mass of oxygen - mass of magnesium oxide

Mass of oxygen = Mass of magnesium oxide - Mass of magnesium