a quick math challenge

[Don Shepherd]

The kids forget it almost immediately. I might teach it to them in 6th grade, and in 7th grade they've forgotten it. They do generally know, however, that all even numbers are divisible by 2.

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.