The molecular weight is between 70 and 85. Let's accidentally pick some quantity in amid, like 78.
7.74% of 78 is how many 'u' there are of H:
78 u * 7.74% = 78 u * .0774 = 6.0372 u
The only hydrocarbon below that has 6 hydrogens is C6H6. We can even check this answer:
Total u = 6C + 6H
= 6(12.01) + 6(1.0079)
= 78.11 u
6(1.0079) / 78.11 * 100 = 7.74%
