The graph below shows the fuel consumption (in miles per gallon, mpg) of a car driving at various speeds (in miles per hour, mph).

(a) How much gas is used on a 400 mile trip at 80 mph?
amount of gas = gallons

(b) How much gas is saved by traveling 60 mph instead of 70 mph on a 600 mile trip?
saved gas = gallons

(c) According to this graph, what is the most fuel efficient speed to travel?
most fuel efficient speed = mph