Let us apply the mass percentage expression.
For any aqueous (or any other type) solution, we shall define the mass percentage (or weight/weight concentration, w/w %) as follows:
where we assume that the whole solution weight (or mass) is water (solute disappears into solvent).
According to the statement, we want to know the mass of solution, and we know the mass of solute (50 g) and the mass percentage (25 %). So:
As we have assumed that all the solution mass corresponds to water, we need