If both solute and solvent masses (mass %) are doubled, the concentration of the solution will:

Mass percentage of a solution is the mass of solute in 100 parts by mass of the solution. It is calculated as: Mass % = (Mass of solute / Mass of solution) x 100. If the initial masses of solute and solvent are x and y, then mass % = [x/(x+y)] x 100. After doubling both masses, it becomes [2x/(2x+2y)] x 100 = [x/(x+y)] x 100. Thus, doubling both solute and solvent masses does not change their mass ratio; therefore, the concentration remains the same.