It's to scare Susan's students. (She's a math teacher.) It's supposed to be easy enough that most of them can get it and feel good about themselves as a result, but hard enough that their first reaction will be fear.
So far, though, no students have shown up at the door. We're mostly getting kids too young to recognize long division when they see it, but we got one teenager who said "20" in place of "Trick or treat" and thereby earned an extra piece of candy. And we got one parent who demanded of Susan, "What does that stand for???" in a wary voice like maybe it was a gang symbol or something. (Which, in this neighborhood, may not be as unreasonable an assumption as it seems.)
Edited to add: Just now we got one of Susan's students, who guessed that the answer was 12. Another kid with her agreed that 12 sounded right, and then the mother of one of the kids told them they were right, that 12 was the right answer! Susan decided to politely say nothing. This kid's mother has made our Halloween truly frightening.