It's quite normal for a cat not to eat when it has changed home. Do you feed the cat the same kind of food it ate before it came to you? If not, it may be a good idea to start off with the same kind of food and change it later on if you don't want to keep it, but like I said your cat is quite normal. If she hasn't eaten anything in three days though you should contact a veterinarian just in case, it's not good for a cat to starve longer than three days.