DQ1 Sets

How many people surveyed believed in at least one of these things?
Answer: The set of all real numbers.
Starting with the intersection of all three circles, we work our way out. Since 11 people believe in UFOs and Ghosts, and 2 believe in all three, that leaves 9 that believe in only UFOs and Ghosts. We work our way out, filling in all the regions. Once we have, we can add up all those regions, getting 89 people in the union of all three sets. This leaves 131 – 89 = 42 who believe in none.