If 20.0 g of NaCl is added to 100.0 mL of water, what is the %(m/m) of the NaCl in solution? Assume the density of water is 1.00 g/mL

% w/w = % m/m = (mass solute/total mass of solution)*100 = ?

mass solute = 20.0 g NaCl
totalmass solution = 20.0 + 100.0 = 120.0
Substitute and solve