DQ 1 - Sets

Because 11 believe in UFO's and Ghosts, and 2 believe in all three; that leaves 9 that believe in only UFO's and Ghosts.
Because 4 believe in UFO's and Bigfoot, and 2 believe in all three; that leaves 2 that believe in only UFO's and Bigfoot.
Because 6 believe in Ghosts and Bigfoot, and 2 believe in all three; that leaves 4 that believe in only Ghosts and Bigfoot.
Adding up all those regions, comes to a total 89 people in the union of all three sets.
So this leaves 160 - 89 = 71 people who believe in none, therefore, 89 people believe in at least one of these things.