9. The purchase price of an air filter for a specific car is $30." The car is driven 1,000 km per month. Gasoline costs $1 per litre. When the air filter is new, during the first quarter of opera- tion the car's mileage is 15 km/litre. As the filter is used, it starts to slowly get clogged with dirt and dust, and the mileage drops as follows: (LO4) Age (quarters) km/litre 1 2 3 4 5 15 14.5 14 13.5 13 What is the economic replacement time for the air filter?