Divisibility Rules for 3 and 9

The video tutorial explains how to determine if a whole number is divisible by 3 or 9. It introduces the concept of divisibility and provides quick methods to check divisibility without long division. 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. For divisibility by 9, the same method applies: sum the digits, and 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. The lesson concludes with a recap of these rules.

What is a quick method to determine if a number is divisible by 3?

• Sum the digits and see if the sum is divisible by 3.

• Divide the number by 2 and check for no remainder.

• Multiply the digits and check if the product is divisible by 3.

• Check if the number is even.

If the sum of the digits of a number is 16, is the number divisible by 3?

• No, because 16 is a prime number.

• Yes, because 16 is an even number.

• Yes, because 16 is divisible by 3.

• No, because 16 is not divisible by 3.

How can you quickly check if a number is divisible by 9?

• Multiply the digits and check if the product is divisible by 9.

• Check if the number is a multiple of 5.

• Sum the digits and see if the sum is divisible by 9.

• Divide the number by 3 and check for no remainder.

Is the number 378 divisible by 9?

• Yes, because the sum of its digits is 18, which is divisible by 9.

• No, because 378 is a prime number.

• No, because the sum of its digits is 18, which is not divisible by 9.

• Yes, because 378 is an even number.

What is the sum of the digits of 378?

• 16

• 18

• 21

• 24