.
It follows directly from the condition that 4 cds cost $285 - $225 = $60.
Hence, 1 cd costs 60/4 = 15 dollars.
Then 5 dvds cost 225 - 5*15 = 150 dollars and 1 dvd costs 30 dollars.
From this point you can easily complete the solution on your own.
The lesson to learn from the solution is THIS:
You NEITHER need solve the system of equations NOR a single equation to get the answer.
With the given condition, you can solve the problem MENTALLY.