Question 891928
The problem description needs more information.


Twenty liters of acid solution of unknown y percent;
Present is 20% acid solution;
Wanted is 12% acid solution.


Let u = quantity of y% acid to mix with
v = quantity of 20% acid solution


{{{highlight_green((u*y+v*20)/(u+v)=12)}}}


THREE unknown variables but just ONE equation.  
The description is missing information for y and for u.
You can solve the equation for v, but finding a value is not possible until you have the other information.