Data:
Strawberry juice: s
6L 20%s
xL 80%
Final: (6+x)L 55%

Solve x:
Simplify:

Leave in one side of the equation the terms with the variable x:

Make substractions.

Divide both both sides of the equation into 0.25:

Then, you shloud add 8.4L of 80 percent strawberry juice to make 55 percent strawberry juice