We have two solutions, call them x and y.
There are 80 liters in the final mix. x + y = 80
Now let's make an acid formula. Since 20% of the x solution is acid, and 60% of the y solution is acid, and 30% of the final solution is acid, then
0.20x + 0.60y = .30(80). ----->. What does this simplify to?
0.2 x + 0.6y = 24 is the acid equation, while x + y = 80 is the solution equation. Solve these simultaneous eqs.
Might help simplify. 0.2x + 0.6y = 24 --->2x +6y = 240 ---->. x + 3y = 120
Subtract x + y = 80 from x + 3y = 120. and get an equation in y only.
x = 60 y = 20. should be your answers.