Divisibility Rules Section 4-1

Bell Ringer #1 Go to page 152 in your books and copy the chart of divisibility rules into your notebook. This is a requirement for your next notebook check so please do not lose this page.

The # 2 Checking if a number is divisible by 2 is the easiest one. If the number ends in an even number (0 , 2, 4, 6, 8) then the number is divisible by 2. Examples: 120, 68, 146, 12 are all divisible by 2.

The # 3 This one can be tricky. For the #3, you must ADD all the digits together and see if that number is divisible by 3. Example: 423 = 4+2+3=9 9 is divisible by 3 so 423 is divisible by 3.

#4 For 4, look at the last 2 digits of a number. If those two digits form a number that is divisible by 4, then the whole number is divisible by 4. Example: 632…..the last two numbers are 32. Since 32 is divisible by 4, 632 is divisible by 4.

#5 5 is another easy one. If a number ends in a 0 or a 5, then the number is divisible by 5. Example: 215, 420, 830.

#6 For 6, you must check the divisibility for 2 and 3 first. If the number is divisible by both of them, then it is also divisible by 6. Example: 192 Check 2: ends in a 2 Check 3: 1 + 9 + 2 = 12 which is divisible by 3. So the whole number 192 is divisible by 6.

#9 The same rule that applied to 3 applies to 9 as well. You must add all the digits together and see if that number is divisible by 9. Example: 5931= 5 + 9 + 3 + 1 = 18 18 is divisible by 9 so 5,931 is divisible by 9.

#10 If the number ends in a 0, it is divisible by 10. Examples: 210, 150, 10, 40.

Example Check if the number 610 is divisible by 2,3,4, and 5. 2: ends in a 0 so ___________ 3: 6 + 1 + 0 = 7 so __________ 4: 1 + 0 = 1 so ____________ 5: ends in a 0 so _____________ So 610 is divisible by ___________

Prime and Composite #’s This is a topic that will be confusing at first, but once you understand divisibility, it will become easier.

Prime and Composite #’s Prime Number: A number that is divisible ONLY by 1 and itself. Examples: 2, 3, and 5 are only divisible by 1 and themselves. Composite Number: Any number that isn’t a prime number. (divisible by more than two numbers)

Examples: Tell whether each number is prime or composite. If composite, list what it is divisible by. 45:____________ 13:____________ 19:____________ 49:____________

Lesson Quiz 1) Tell whether 462 is divisible by 2, 3, 4, and 5. 2) Prime or Composite 23 48 31 18