Question 468016
Total cost can be broken down into fixed cost and variable cost.
Fixed cost is independent of the variable x, or in other words doesn't depend on the number windows replaced.
Variable cost is dependent on x and is a unit cost or cost per window.
For this problem the service call represents the fixed cost because they charge the same $600 no matter how many windows are replaced.
The variable cost is the $400+Z which is the cost for each window.
Since x represents the number of windows to be replaced then the cost of replacing x windows is as follows:
C(x) = 600 + (400+Z)x