How did your teacher in second grade taught you subtraction?
Your teacher used words. It's as simple as that. You don't write any formulas in there. If you are bent on making it a little mathematically because that's what your character is about, as indicated by your comments, you can easily do that.
(Direct Cost / (100 - Desired Profit)) * 100
To get a first estimate I have to get an idea of the direct costs involved and my desired profit. There are also some fixed costs that I have to be aware of. This is going to be a bit tricky, but if I subtract my desired profit from the 100$ fixed costs I will have the basis to...
Here we see one problem with your initial approach: I have no idea what you are talking about. The formula doesn't make any sense without any explanation, but using words you can see that my description doesn't make any sense. But this is still a bit awkward and you should probably skip over some of the stuff. Let's try a different approach:
Wait... that doesn't work. Let's see... First I have to define my desired profit, let's say 200$ per night, and my fixed costs, roughly 10$ per night. Now I can find out how many customers I need by looking at the average price in the region and then I - holy crap?! So many?! This is going to be rough...