Divisibility Rules for 3 and 9

The video tutorial explains how to determine if a whole number is divisible by 3 or 9 using simple tricks. For divisibility by 3, sum the digits of the number; if the sum is divisible by 3, so is the number. For example, 21 is divisible by 3 because 2 + 1 = 3, which is divisible by 3. Similarly, for divisibility by 9, sum the digits; if the sum is divisible by 9, the number is too. For instance, 378 is divisible by 9 because 3 + 7 + 8 = 18, and 18 is divisible by 9. These methods provide quick checks without performing long division.

What is the rule for checking if a whole number is divisible by three?

How do you determine if the number 21 is divisible by three?

Explain why the number 79 is not divisible by three.

What is the method to check if a whole number is divisible by nine?

Using the number 378, demonstrate how to check its divisibility by nine.