CM3 = ML, they are equivalent. No need to convert.
Therefore you have 54.2ml of solution, which weighs 49.6g total.
Water is 1gram/1ml. They give you ethanol’s density as 0.789g/ml.
Pure ethanol, 200 proof, is “everclear”. They cut it with H2O to make the lower proof contemporaries, like vodka, etc in the different % ethanols — add more H2O, % ethanol decreases.
(This is also why adding H2O to parent’s liquor and putting in freezer = ice, when ethanol’s freezing point is -180C).
But I digress.
ethanol: 54.2ml (0.789g/ml) = 42.76grams ethanol.
49.6-42.76 = 6.84, therefore the total H2O is a bit over 5ml
42.76/6.84 = 6.25, therefore 6.25% of the total is H2O, which means your proof is still very close to 200. You’ve only cut it by 6%.
Unfortunately, I don’t know how to figure out the proof from here, but at least I got you 1/2 way!
Maybe if you multiply 200 by 0.0625 (which is 12.5)… 200-12.5 = 188.5 proof, therefore, 94.25% alcohol? (Guessing, but the answer sounds reasonable!)