Systems of Linear Equations: Antifreeze Drain and Replace Problem 
The radiator in your car contains 4 gallons of antifreeze and water. the mixture is 45% antifreeze. How much of the mixture should be drained and replaced with pure antifreeze in order to have a 60% antifreeze solution? Setup as a system of linear equations using two variables, and round to the nearest tenth of a gallon.

I'm having trouble setting this up as a system of linear equations in two variables. 
So far I have the following: 
Let $x =$ amount of pure antifreeze added
Let $y =$ amount of mixture/solution drained
Since 45% of the 4 gallons is already antifreeze, then 1.8 gallons out of the 4 is antifreeze. 
We're trying to get a solution that is 60% antifreeze, so we should end up with 2.4 out of the 4 gallons being antifreeze in the radiator. 
There's already 1.8 gallons of antifreeze in the car, and we'll be adding $x$ amount of pure antifreeze to that to get to 2.4 gallons: $1.8 + x = 2.4$ 
But before anything can be added some of the solution already in the radiator must be drained. Whatever amount is drained 45% of it will be antifreeze, so my first equation is this: $1.8 + x -0.45y = 2.4$ 
Simplify: $x -0.45y = 0.6$ 
I don't know how to set up the second equation. 

x = amount drained from radiator.
0.45(4 - x) + x = 0.6×4.
