This problem may easily solved by applying the conservation of mass, which states that the total mass before and after a change is constant because mass can neither be created nor destroyed.
We know that water consists of only hydrogen and water, and that there are no other reactants except hydrogen and oxygen. Thus:
Mass of reactants = Mass of product
mass of hydrogen + mass of oxygen = mass of water
4 + mass of oxygen = 36
mass of oxygen = 32 grams
