The petrol tank of Jenny's car holds 50 litres of petrol. A warning light comes on when only 5 litres are left. She always fills it as soon as she can after the warning light comes on. The car travels 100km on 8 litres of petrol. Last time she put petrol in, which she was able to do immediately after the light came on, she only had $6 with her so she spent it all on fuel at 60c per litre. She then drove 50km and saw a new garage offering petrol at 50c per litre - an offer too good to miss. She filled the tank completely and paid with her credit card.