We let the amount of 32% acid solution added to be X , Firstly, 40 of 0.12 conc. , = 4.8L of acid, and 35.2L non acid For ever 1 L of 32% acid we add in we get 0.32L of acid and + 0.68L non Acid, So we formulate an equation (4.8+0.32X) / (40+X) = 16/100 (4.8+0.32X) = 0.16(40+X) =6.4+0.16X 0.16X = 1.6 X =10 Hence she used 10L of 32% Solution acid.