Molecular formula/Mass spectrometry please help

54,2 % oxygen means there are four oxygen atoms as 118 x 54.2/100 = 64 = 4 oxygen atoms

The relative mass is 118 and subtracting your four oxygen atoms (64) from this leaves 54

Each carbon is 12 therefore 3 carbon atoms (36) would need 18 H atoms (clearly impossible)
5 carbon atoms = 60 units - also impossible

Therefore there are four carbon atoms (48) and 6 hydrogen atoms

Thank you so much! But why is 3 carbon atoms and 18 hydrogens impossible? - 36 and 18 add up to 54 and there might be a structure that fit this?

You're 'aving a laff!

Carbon has a maximum valency of 4 and hydrogen 1, the most hydrogen atoms possible for 3 carbon atoms is 8.