Division Rule for 7

It does exist! In a short presentation, I explain how to determine if a number is divisible by 7. This method I figured out myself, although it is kinda lengthy.

There are other rules for 7 around the internet. Some are more formulaic but may require multiple iterations. I do like this one because it can result in a number whose divisibility is pretty evident, just the setup is a little chaotic.

Category:

Education

Tags:

• Math
• Divisibility
• by
• seven

[mathgeek37]

By the way, Wikipedia's Divisibility Rule article has a concise description on why the Divisibility by 7 methods work.

Nice video.

[mathgeek37]

There is a simpler method...

Cast off the number in the ones places and then subtract the double of the cast off digit from the new number and repeat until you have a multiple of 7.

For example....999999

99999 - 18 = 99981

9998 - 2 = 9996

999 - 12 = 987

98 - 14 = 84

8 - 8 = 0

It works because 50 is congruent to 1 (mod 7). It has something to do with the fact 5 + 2 = 7. The other way to write the rule is to cast off the last digit and 5 times the digit to the remaining number.