It is given that the car uses 8 liters of petrol to travel 100 km. Initially, $72 is used to buy petrol at $1.5 per liter. With $72 the number of liters of petrol bought is 72/1.5 = 48. Using this the car can travel for 48/8*100 = 600 km.
After traveling for 600 kilometer, 48 liters of petrol is used and it is time to refuel.
The problem requests the rule of three to determine the number of Km covered along the trip.
Suppose that the maximum capacity of your tank is of 55 l.
We'll apply the rule of three:
8l are necessary for 100 Km
55 l are necessary for x Km
x = 55l*100Km/8l
x = 55l*25Km/2l
x = 687.5 Km
So, if your tank capacity is of 55l, you'll have to travel 687.5 Km to empty your tank.