How can u tell if a number is divisible by 3?

To check for divisibility by 3 just check if the sum of all the digits in the number are divisible by 3. If so, the number itself must also be divisible by 3. For example, is 1,529 divisible by 3? Well, the sum of the digits of 1,529 is 1+5+2+9=17. Since 17 is not divisible by 3, we can conclude that 1,529 is also not divisible by 3. How about 1,530? Well, this time the sum is 1+5+3+0=9. Since 9 is divisible by 3, we know that 1,530 is divisible by 3 too.

Idon’t understand sorry