The easiest way to solve this is to work with two equations:
X + 4000Y = $5000 (1)
X + 2000Y = $4000 (2)
Where X is the fixed portion of the water bill and Y is the cost of water per car.
Solve for X for each equation:
X = $5000 – 4000Y (3)
X = $4000 – 2000Y (4)
Therefore:
$5000 – 4000Y = $4000 – 2000Y (5)
Solve for Y:
Y = $0.5
Substitute into equation1:
X + 4000(0.5) = $5000
X = $3000