And the winner of this month's Genius of the MonthTM Award is my boy Sammy Greenspan. I posted the challenge at about 4am Pacific Time, and Sam responded about 4:15 am.
The answer was 18. I'll post a screenshot of Sam's explanation, since he went to all the trouble of making division lines with hyphens and stuff.
One could argue that he had an unfair advantage because we're the only two people who are both up and doing SAT problems at 4:15 am on a Friday morning. Some might argue that this is sad; I would argue that it's awesome. Awesomely, sadly awesome.
Runner-up explanations coming soon.