Marie mixes 40 liters of 12% acid solution with a 32% acid solution to make a 16% acid solution. how many liters of 32% solution did she use?

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.