Well, the numbers are used that way, because the problem gave does conditions (e.g. the densities, volumes, and masses); I assumed that these conditions were given, because they're needed in the calculation; isn't this how problems are usually solved (e.g. the question states the conditions and you take those to solve the problem)? If I looked for something other than what's given in the problem, than either I'm going on a tangent or the person creating the problem is creating a bate and switch; but, yes, as I suspected, the answer in the back of the book must be wrong.
If you used a density table, how did you know which temperature to assume for a specific density? You could pick every temperature there. Why do we have to work out the problem but you not? Although not required, it would aid greatly if you extended the courtesy to show how you're deriving the answer. Also, if my calculations are off, I'll need you to be more specific and telling me. But, my answer took each of the masses, got the total mass, than calculated the mass percent of one of the variables. But, each of our answers are close to each others while the textbook answer is way off; if you used a density at a different temperature, than the density would be different from the one stated in the textbook and would explain the differences in our answers (e.g. we all used different densities).