So in my book, it has the following statements which are meant to help you do prime factoring more quickly without a calculator. (I assume.)

1. If the sum of the digits of a number is divisible by 3, then the number is divisible by 3.

So this leads me to assume that it works with 2 digit through 10 digits through as many digits as your Algebra math problems can have.

Q1) Are there any limitations to statement 1?

There are others as well.

2. If a number is even and the sum of its digits is divisible by 3, the number is divisible by 6.

3. If the sum of the digits of a number is divisible by 9, then the number is divisible by 9.

Have a great day or night as the case may be!