Be careful about calculating about the mass of solution as you always have to take the sum of masses of solute and solvent to calculate the mass of solution.
Mass of sodium chloride (Solute) = 36gm
Mass of water (solvent) = 100 gm
Mass of solution = 36+100 = 136 gm
Concentration = (Mass of Solute / Mass of Solution) * 100
Concentration = (36/136)*100 = 26.47%