Does anyone know of a quick test for divisibility by 7? I've mastered the tests for divisibility by 2, 3, 4, 5, 6, 8, 9, 10, 11 and any composite number beyond 11. I know that a test for 7 exisits and I'd like to know what it is. Thanks.

A quick Google search turned up a couple of divisibility tests for 7 on this page:

http://www.jimloy.com/number/divis.htm