explain how to find the mass of solute in the solution: 50.0g of solvent in a 10.0% NaCl solution?

% by mass?

10% means 10 g solute in 100 g solution.
That's 10 g solute in (10g solute+90 g solvent)
0.1 = (xsolute)/(xsolute + 50g solvent)
Solve for x.