Rule for 9

The sum of the digits is divisible by 9,

eg. the number is 18972, add all the digits up. 1+8+9+7+2=36

36 is divisible by 9 so that means 18972 is divisible by 9

Here are some more examples,

eg. 4+6+3+2+1+5+6+9+7+2=4+5= 9

9is divisible by 9

eg. 8+1+9+6+3+7+2+4+5 = 4+5 = 9

9 is divisible by 9

eg. 3+7+8+3+6+5+4+7 = 4+3 =7

7 is not divisible by 9