What does it mean to have a false positive? You know that a a test says you have the fever if you actually have the fever 80% of the time. It also says you don't have the fever when you don't actually have the fever 90% of the time. Consider now that you don't actually have the fever but it gives you a positive test anyway. You should have something like
[Not Sick] * [Positive Test] + [Sick] * [Negative Test] = [False Positive]
In other words, the given how many people are sick or not sick and the chances that the test gets it wrong, you should be able to figure out its false positive rate. I got the correct answer, and from the expression above, so should you. The key here is understanding why it is set up that way. Do you see why?