There are 20 gallons of 20% butterbeer solution.
Assume there are x gallons of 80% solution.
So total amount of the solution will be (x+20) gallons
The final solution is 50%.
So, we can set up the equation as:

.
Thus, there will be 20 gallons of 80% butterbeer solution that is needed to be mixed with 20 gallons of 20% butterbeer solution.