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?

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

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Explain why the number 79 is not divisible by three.

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

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

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