Let x l be the amount of 40% alcohol solution and y l be the amount of 80% alcohol solution.
1. Marisol needs 2 liters of a 50% alcohol solution. Then
x+y=2.
2. In x l of 40% alcohol solution is 0.4x l of alcohol, in y l of 80% alcohol solution is 0.8y l of alcohol and in 2 l of 50% alcohol solution is 0.5·2=1 l of alcohol. Then
0.4x+0.8y=1.
2. Solve the system of equations:

From the first equation

Substitute it into the second equation:

Then

Answer: Marisol should combine 1.5 l 0f 40% alcohol solution with 0.5 l of 80% alcphol solution.