a quick math challenge

[Waon Shinyoe]

(12-28-2013 10:06 PM)Les_Koller Wrote:  I teach 9th grade Alg 1, and am happy to report approx 80% of my students rember the 2, 3, and 5 rules. Cannot figure out why they can't remember the by 6 rule!

Excuse me but what's the 2,3, and 5 rules? I think a 9-digit number contains 1~9 can be divided by 3 because (abcdefghi) mod 3 = (1*9 + (0+1+2)*3 ) mod 3 = 18 mod 3 = 0
(I'm a high school student in P.R.China.)