The decomposition of water takes place in the following way -
We find that the ratio of mass of hydrogen to that of oxygen in water is 1:8 , so when water decomposes it leads to production of hydrogen and oxygen in 1:8 proportion.
So this means that decomposition of 90 kg of water produces 10 kg of hydrogen and 80 kg of oxygen and the total mass before and after decomposition is 90 kg.
This follows the law of constant proportions.
Thus 80 kg of oxygen can be produced by 90 kg of water.