Question

Which rule can be used?

Answer 1

Thes first rule is correct. 

Another way of doing it is:

Sum the digits of the number. If the result is divisible into nine, then the original number itself will also be.

Example:

18 = 1+8 = 9. 9 is divisible into 9.

27 = 2+7 = 9. 9 is divisible into 9.

Wish you luck!