"In a survey of 185 university students, 91 were taking a history course, 75 were taking a biology course, and 37 were taking both. How many were taking a course in exactly one of these subjects?"
I know the answer is 92 from the back of the book. I know also that when I draw out a Venn diagram I find that 37 take both, 54 take History, 38 take Biology. In my text book it says that $n(A \cup B) = n(A) + n(B) - n(A \cap B)$ yet, using this does not work. Can someone please explain why?