What is the mass % of carbon in 1 mole of hydrocarbon C_{x}H_{y}, in terms of x?

Hint: think about how you calculate the molecular weight of a hydrocarbon where you know what the formula is. can you think of a way to do that in reverse?

Although if you think carefully about it, you'll realize that for the series of alkanes (for example), after you get past the first few, the mass % of carbon in all of them is about the same. This should make sense to you. As the chain gets longer and longer, the ratio of carbons to hydrogens gets closer and closer to 2:1, which is the ideal ratio in the infinitely long chain. In finite chains, the only difference is the terminal CH3 groups... and the longer your chaain gets, the less important those become. A molecule with only carbon and hydrogen with a 2:1 hydrogen to carbon ratio has a % carbon by mass of ~12/14 or... ~85.7%

So actually you can just about pick any long chain alkane here if it helps you and you'll probably get pretty close to the right answer. In reality kerosine is a mixture of high molecular weight hydrocarbons, so there's no exact formula.