What is the rule for dividing by 7?

Answer 1

I assume you mean "divisibility rule". Separate the last three digits, and subtract it from the remaining digits. For example:

45003: separate 45 and 003, and subtract 45 - 3 = 42. Since 42 is a multiple of 7, the original number is a multiple of seven.

Note that this rule works for 7, 11, and 13. The reason it works is related to the fact that 1001 = 7 x 11 x 13.