According to Dalton's Atomic Theory, the individual elements making up the compound has a fixed ratio. This law is known as the Law of Definite Proportions. Obeying this law, the number of hydrogen to oxygen should be a constant, so we can use ratio and proportion.
The ratio can be determined from the molecular formula of methyl tert-butyl ether: C5H12O. This is the correct molecular formula of the compound. To check, try searching it in the internet. Its structural formula is drawn in the attached picture. From the molecular formula, we can see that there are 5 carbons in every 12 hydrogens and in every 1 oxygen. So, the hydrogen to oxygen ratio is 12/1. Applying ratio and proprotion,
12/1 = 0.0723 moles of hydrogen/x, where x is the amount of moles oxygen. Solving for x,
x = 0.006025 moles of oxygen
