Question

When 5.6 grams of NaCl are dissolved in enough water to create a 125-gram solution, what is the solution's concentration, expressed as a percent by mass

Answer 1

You need to divide the mass of the solute by the mass of the solution.

The question could have been trickier, if they only mentioned the masses of the solute and the solvent. In that case, you would have to manually add the solute to the solvent to get the total solution mass. As it is, they did that for you:

(5.6 g)/(125 g) = 0.0448 = 4.5 % NaCl in water