Two friends decided to embark on a motorcycle trip uphills. They filled their motorcycle tank to a full capacity of 18 litres of petrol. As soon as they started out, their fuel tank developed a snag and started leaking petrol. After having travelled for 4 hours exactly at a speed of 100km/hr, they decided to take a break for an hour. The leak continued at the same rate as before. After the break, they again travelled for 2 hours at a speed of 75 km/hr and then their bike ran out of petrol. The average of their motorcycle is 50 km/ltr. How much did they lose due to the leak while they were travelling?

1) 6ltrs
2) 7ltrs
3) 8ltrs
4) 9ltrs