Marie mixes 20 liters of 12% acid solution with a 36% acid solution to make a 20% acid solution. how many liters of 36% solution did she use? 10 liters 12 liters 13.7 liters 16 liters

Marie has two solution of the same acid with different concentration 
 Solution A 12% - 20 liters
 Solution B 36% - Unknown 
 And she wants to obtain a new solution C with at 20% 
 According to the proporcionality of the dissolution In mathematical terms will can write the following equation: 
 quantity of A * concentration A + quantity o B * concentration B = (qty A + qty B) * concentration C 
 20*0.12 + x*0.36 = (20+x) * 0.2 
 being x the unknown quantity of the solution B solving the equation: 
 2.4 + x*0.36 = 4 + x*0.2
 x*0.36 - x*0.2 = 4 -2.4
 x*0.16 = 1.6
 x = 1.6/0.16 = 10 
 So the volume of solution B at 36% is 10 liters

Ithink the answer may be -7 by when dividing both sides by -3