a quick math challenge

[Maximilian Hohmann]

(12-27-2013 04:43 PM)Don Shepherd Wrote:  Regarding adults, I suspect that if you took a man-on-the-street poll, you'd probably find fewer than 5% would know the divisibility-by-3 rule.

Somewhere back in my head I might have found a 40 year old memory of that rule. If I would have searched for it, because it probably wouldn't have crossed my mind to apply it to this challenge in the first place...