Mass percent asks a plain question: of everything in this bottle, what share by mass is the solute? $c = \dfrac{m_s}{m_t} \times 100\%$ — the solute's mass over the mass of the whole solution. Stir 25 g of salt into 225 g of water and the bottle holds 250 g of solution — the salt is $\dfrac{25}{250} \times 100\% = 10\%$ of it, not $\dfrac{25}{225}$ of the water.

That is the whole trap of this lesson, so hear it twice: the solute is PART of the solution it sits in. The denominator is solute plus solvent, added before anything is divided. Divide by the water alone and your percent floats high — a small lie at 5%, a scandalous one at 40%. The solution's mass is the sum; the sum goes downstairs.