According to the balanced equation for synthesis of water, hydrogen is used up twice as fast as oxygen -- we need two moles of H2 for every one mole of O2 when creating water.
Since we have 20 moles of oxygen, this means we would need twice as much hydrogen, or 40 moles, to use it all up. There is only 30 moles of hydrogen, which means that all the hydrogen will be used before the 20 moles of oxygen is used. Hydrogen limits production in this case.
