In order to answer this question, we are going to look at the molar ratio between H2O and H2, we can see that by analyzing the numbers in front of the compounds, and we can see that the molar ratio is 2:2, this means that for every 2 moles of water decomposing, we will end up with 2 moles of H2 being produced, therefore the same amount of moles for H2O will be the same amount of moles of H2, which means that if we have 100 moles of H2O being decomposed, we will also have 100 moles of H2 being produced.