HAPPY CAMPERS (MODERATELY DIFFICULT)

You just bought nine beautiful lakeside campsites, which you can rent out to campers with tents for $20 per campsite per evening. You can also upgrade them with electrical hook-ups: this will cost you $60 per campsite but will allow you to rent them to visitors for $40 per evening. 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?