A compound used in glue has a composition of 54.5% C, 9.1% H, and 36.4% O.
At 100 degrees Celsius and 1.00 atm, the compound is a gas and 0.345 g occupies a volume of 120.0 ml.
What is the molecular formula of the compound?

The first sentence gives the ratio by mass of the elements involved. You can determine the relative numerical quantities by applying the atomic mass of each.

From the second sentence you can determine the molar quantity it is referring to and then the molecular mass.