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[-] Dagwood222@lemm.ee 30 points 10 months ago

Any number where the individual digits add up to a number divisible by '3' is divisible by 3.

51 = 5+1 = 6, which is divisible by three.

Try it, you'll see it always works.

[-] letsgo@lemm.ee 8 points 10 months ago

There are tricks like that for a lot of numbers. For 7, chop off the last digit, double it and add it to what's left. Repeat as required. If the result is divisible by 7 then the original number was. eg: 356 -> 35+12=47 not db7. 357 =>35+14 both db7 so we don't even need to do the add.

[-] diverging@lemmy.ml 3 points 9 months ago

double it and ~~add it to~~ subtract it from what’s left

[-] BruceDoh@sh.itjust.works 1 points 9 months ago
[-] sockenklaus@sh.itjust.works 3 points 9 months ago

You clearly mean:

14: 1 + 8 = 9 (not db 7)

Someone else in this thread correctly stated:

"Chop of last digit, double it and subtract from what is left"

14: 1 - 8 = -7. (dB 7!)

Math is awesome, I didn't know this trick!

[-] BruceDoh@sh.itjust.works 1 points 9 months ago

Yes, that's what I meant. The subtract rule works.

[-] Zacryon@feddit.de 5 points 10 months ago

One of the reasons why I love the number 3. There are other neat digit sum tricks, see for example for the numbers 1 to 30 here: https://en.m.wikipedia.org/wiki/Divisibility_rule

[-] Dagwood222@lemm.ee 7 points 10 months ago

They didn't teach stuff like this in school, which is silly. This is the kind of thing that a kid would eat up. It's like they wanted to make sure people hated math.

[-] steeznson@lemmy.world 4 points 9 months ago

My experience of maths in high school was being taught a trick or method to solve a really specific type of problem every week. Sometimes the method would build off something we'd learnt the previous week.

The whole thing was bottom-up learning where you get given piecemeal nuggets of information but never see the big picture. They completely lost me at around the age of 15. I eventually came back to maths later in life after studying formal logic in my philosophy undergrad degree.

[-] Zacryon@feddit.de 2 points 9 months ago

I guess I was one of the lucky few who learned this in elementary school. And later again.

[-] Dagwood222@lemm.ee 1 points 9 months ago

I first saw the nine times finger trick in the movie 'Stand And Deliver.' I remember seeing it written out on a blackboard at some point, but never the finger trick.

https://youtu.be/-WOgLltWhgg

[-] PipedLinkBot@feddit.rocks 1 points 9 months ago

Here is an alternative Piped link(s):

https://piped.video/-WOgLltWhgg

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[-] Laukidh@infosec.pub 3 points 9 months ago* (last edited 9 months ago)

I knew that worked with 9. Hmm, does it work with 6?

Doesn’t look like it.

[-] AEsheron@lemmy.world 2 points 9 months ago* (last edited 9 months ago)

Technically it does work for 6, more literally, still aiming for 3, not 6. That's half of it, if the starting number is even and divisible by 3 then it is also divisible by 6.

this post was submitted on 22 Oct 2023
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