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:

What are our unknowns? We need to find the volume of the 40% solution needed (call it ), and the volume of the 55% solution that is needed (call it ). We know the total amount should come to 15 liters, so this gives us one equation:

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.

Again, what are the unknowns? We have two types of radios, each selling a certain number of units (call the number of cheap radios , and the number of the others ). We know that 25 radios were sold in total, so that gives

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.