We are given:
Mass of solvent (water) = 100 grams
Mass of solute (NaCl) = 20 grams
Mass of the Solution:
Mass of Solution = Mass of Solute + mass of Solvent
100 + 20 = 120 grams
Mass Percent of NaCl:
We know that
mass percent of NaCl = (Mass of NaCl / Mass of Solution) * 100
replacing the variables
Mass% of NaCl = (20 / 120)*100
Mass% of NaCl = 100 / 6
Mass% of NaCl = 16.67%