This a question that I can`t answer as an expert in my field but I will as a friend.

in the case you described the "rent" you should sharge her depends on several thigs.

1. your need of this rent money? Do you need that little money for rent she could give you ?

2. if she is contributing with a bill or so then, do you think she is paying for what she is using in this apartment.

3 if you want to be by the book, a 3ed of her income should go to apartment rent so you can ask her for $240

Hope that helps and thanks for asking.

when you have a chancse take a look at my site and tell me whet you think.

www.ApartmentsRentRebate.come and it is an apartment rental that gives Apartment Rebate for people that us it`s free service.

Thanks