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 new insight or understanding did you gain from this video?