John filled the gas tank of his car with 32.4 litres of gas just before starting on a trip. He noticed that the odometer read 30803 km as he left the gs station. When he stopped for lunch, he saw the next palindrome on his odometer. He then filled up with 42 litres of gas at the palindrome after the one he saw at lunch. What was the averages number of litres per 100 km used by his car during the trip?