In response to a message to All on ...
RC>The professor challenged me to find one for 7.
Sorry this is so late - there *is* a rule, but it has been so long 
since I've used it that I'm not sure if I remember it correctly.  
Here's ow I *think* it goes...
Start with a number, cross off the last digit, subtract twice that 
deleted last digit from the remaining portion of the number.  Repeat 
the process.  as needed.  if the remainder is 0 or any other 
multiple of 7, the number is divisible by 7.
Example...
 2156  Start with this number, cross off the six, leaving...
 215  but now subtract 2 x 6 (12)...
 -12
 ___
 203  Repeat the process - drop the 3...
 20   and double the 3 (6) to subtract...
 -6
 __
  14  remainder is 14, a multiple of 7, so 2156 is divisible by 7.
Sorry I'm so far behind in the mail or I might have gotten this to 
you on time.
Chuck Beams
cbeams@dreamscape.com
http://www.dreamscape.com/cbeams
___
* UniQWK #5290* Newt: Species of salamander-green with red spots, lives in 
caves
--- Maximus 2.01wb
---------------