Divisibility rules for 3, 4, 6, 8 and 9
We briefly review the easy divisibility rules for 2, 5, 10, and 100. Then I explain in detail, and with examples, the divisibility test for 3 (add the digits; if the digit sum is divisible by 3, so is the number) and the similar test for 9. The rule for 4 says that if the last digits of the number are divisible by 4, then so is the number itself. The rule for 6 is that if a number is divisible by both 2 and 3, then it is divisible by 6. For 8, I use a version stating that if half of the number is divisible by 4, then the number itself is divisible by 8.
This math lesson is meant for 5th or 6th grade, when students study factors, divisibility, and prime numbers.
See also
Math Mammoth Grade 5 curriculum