Algebra 2 Math Problem?

For her chemistry experiment, Marisol needs 2 liters of a 50% alcohol solution. The lab has two containers, one with 40% alcohol solution and the other with 80% alcohol solution. 

1. Write a system of linear equations that you can use to determine how many liters of each type of alcohol solution Marisol should combine to get 2 liters of a 50% alcohol solution. Be sure to define your variables.