7 A bus started from Delhi for Mussoorie with 50 passengers on board. 20 passengers got off at Dehradun and the bus proceeded with the remaining passengers. In the evening the same bus left Mussoorie with 50 passengers, 10 passengers got off at Dehradun and the bus resumed its journey with remaining passengers for Delhi. The distance between Delhi and Dehradun is 280 km. and between Dehradun and Mussoorie it is 20 km. Compute the cost per passenger km., if the total cost of running the bus comes out to be Rs. 5,000