You can do it! Figure out your knowns and unknowns, and just find some way to relate them mathematically (it sounds like the latter step is where you're having difficulty). This should get you off the ground:
But we also know that the final solution should be 50% alcohol, so this gives us our second equation: Because there is liters of alcohol in the liters of the first solution, and liters of alcohol in the liters of the second solution, the final solution will have a total of liters of alcohol, which should come to 50% of the total 15 liters:
You now have two equations with two unknowns.
but we also know that the total revenue brought in from the radios was $2495, which gives us
Again, you will just need to solve this system of linear equations.