SOLUTION: 21. (See Week 4 Lecture page “Solving Rational Inequalities” for a starting point.) Suppose that a factory’s cost (in dollars) for making x cars is C(x) = 7020x + 8460 Then t

From the condition, this phrase "the average cost per car is no more than $7200"

means that an inequality takes place


 <= 7200.    (1)


And they want you solve this inequality for x.  


To solve it, multiply both sides by x.  Since x is positive, by its meaning, you can do it without problems. You will get


7020x + 8460 <= 7200x.


Simplify it by subtracting the number 7020 from both sides. You will get

8640 <= 7200x - 7020,   

8640 <= 180x,

x >=  = 48.


So,  x >= 48  is your solution and is your answer.