A camera rental company charges a fixed amount plus a fee based on the number of days for which the camera is rented. The graph below shows the amount paid for a different number of days to rent the camera: A graph titled Camera Rentals plots the Number of Days on the x axis and the Amount in dollars on the y axis. A straight line joins the ordered pairs 0, 40 and 4, 120. What is the fixed amount charged? $20 $30 $40 $80