How can you quickly check whether a number is exactly divisible by 9?
According to mathematicsnotebook:
A number is divisible by 9 if the sum of its digits is divisible by 9. For large numbers this rule can be applied again to the result. In addition, the final iteration will result in a 9.
More Info:
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What are your thoughts on this subject?
47 Comments

Louella Wilson
I've always thought it a useless fact. Interesting, but useless. I can divide by 9 as quickly, or quicker, than adding digits before dividing that sum by 9.
8
Oct 31, 2019 12:56AM

Georgina Morrison
SUM is the answer to an addition question and PRODUCT is the answer to a multiplication questionHenry Ford 1889,
0
Jun 25, 2022 2:13AM

Henry Ford 1889
When I was at school, sum and product were one and the same.
0
Jun 19, 2022 11:03AM

Chris Brown
Or if the separate digits add up to 9. 9x2=18; 9x3=27 etc. Also, for any number added to 9, the digits add up to the number added on: 9+1=10; 9+8=17...
0
May 31, 2022 6:25AM

LevibatteryCSA (Dave)
I don't use Math any more.
1
Jan 17, 2022 5:46AM

Brian Reynolds
I recently watched a TV program called "The human calculator". The man was incredible. He would stop strangers in the street and after a few questions would tell them their birth dates. He then demonstrated all sorts of calculations with the number nine. In one scene he was in a supermarket and tallied a large customer bill in a matter of seconds. An amazing program and I hope there are more episodes to come.
2
Oct 9, 2021 12:35AM

Nikki Proctor
that is how i taught my girls their 9 x tables. everything adds up to nine (ie 4X9=36, 3+6=9)
0
Aug 11, 2021 4:06AM

jonbroberts
Same rule supplies with 3. If the sum of the digits is divisible by 3 then the number is also divisible by 3.
2
Jul 2, 2021 8:19PM

Richard Rubright
Actually, the sume of the digits must be exactly 9. If more than 1 digit keep adding until there is only the digit 9.
2
Sep 2, 2020 7:28PM

james parrillo
Joseph A. Robitaille, The question asked another way to tell if a number may be divided by 9 evenly. It is possible to add all the digits in the number and then tell if the sum of the digits is also divisible by 9. Dividing the original number by 9 and adding and dividing that number by 9 both work to tell if the number is evenly divisible by 9. Thank you, Christine, James' wife
0
Mar 12, 2020 12:18PM

james parrillo
docluci, Be well. God bless you. Christine, James' wife
0
Mar 12, 2020 12:14PM

james parrillo
Michael Conway, It is so good you are playing. More of your guesses may be right, and you will learn either way. Thank you, Christine, James' wife
0
Mar 12, 2020 12:13PM

Deb Schlawin
Learned this in middle school as casting out nines! Has come in handy many times.
1
Jan 30, 2020 1:54AM

docluci
GOt this answer
1
Jan 23, 2020 9:25PM

docluci
Used a hint. Am in ICU and doing things is difficult
0
Jan 23, 2020 9:22PM

tbear
tesla alset, got your back, Tesla! 😊 👍
0
Jan 15, 2020 10:32AM
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