19. Some drivers try to estimate their annual cost of repairs $c in relation to their average speed of driving s km/h using the equation
c=6s + 50. Draw the graph for 0 <s<160. From the graph find:
a) the estimated repair bill for a man who drives at an average
speed of 65 km/h
b) the average speed at which a motorist drives if his annual
repair bill is $300
c) the annual saving for a man who, on returning from a holiday,
reduces his average speed of driving from 100 km/h to 65 km/h.