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.