It got me, TBH. When I read he bought the horse back at $80, it threw me off because (in my head) I was presuming he didn't have $80. He sold the horse for only

It got me, TBH. When I read he bought the horse back at $80, it threw me off because (in my head) I was presuming he didn't have $80. He sold the horse for only $70. The "riddle" didn't say that's all he had, but it shifted the way I was thinking about it mid-question in a way that led me to thinking about it the wrong way. Of course, that doesn't explain how people got wrong answers other than mine, but I do think the wording and framing is significant here, which is probably why this specific "riddle" is the one that is effective.