Happy Campers

You just bought nine beautiful lakeside campsites, which you can rent to campers with tents for $20 per campsite per night. You can also upgrade the sites with electrical hookups. This will cost you $60 per campsite but will allow you to rent to Rvers for $40 per night. Suppose you can always fill your campground to capacity. If you’re starting without any cash on hand, how many nights will pass before you’ll be able to upgrade all nine sites?