# Amazing math! Forward to all your friends and you will be rewarded!
**URL:** https://boards.straightdope.com/t/amazing-math-forward-to-all-your-friends-and-you-will-be-rewarded/422481
**Category:** Factual Questions
**Created:** [October 12, 2007, 4:05pm UTC](https://boards.straightdope.com/t/amazing-math-forward-to-all-your-friends-and-you-will-be-rewarded/422481 "2007-10-12T16:05:06Z")
**Posts on this page:** 10
**Page:** 1
### Author:  [@tdn](https://boards.straightdope.com/u/tdn)
#### Post date: [October 12, 2007, 4:05pm UTC](https://boards.straightdope.com/t/amazing-math-forward-to-all-your-friends-and-you-will-be-rewarded/422481/1 "2007-10-12T16:05:06Z")
[QUOTE=annoying coworker forward]
Here is a math trick so unbelievable that it will amaze you.
1. Grab a calculator (you won’t be able to do this in your head)
2. Key in the first three digits of your phone number (not the area code).
3. Multiply by 80
4. Add 1
5. Multiply by 250.
6. Add the last 4 digits of your phone number
7. Add the last 4 digits of your phone number again.
8. Subtract 250.
9. Divide number by 2
Do you recognize the answer??
[/QUOTE]
How does this work?
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### Author:  [@Liberal](https://boards.straightdope.com/u/Liberal)
#### Post date: [October 12, 2007, 4:10pm UTC](https://boards.straightdope.com/t/amazing-math-forward-to-all-your-friends-and-you-will-be-rewarded/422481/2 "2007-10-12T16:10:56Z")
According to [this](http://groups.google.com/group/misc.consumers.frugal-living/browse_thread/thread/5e34f264cbed3f6e), it’s just an application of commutativy, distributivity, and associativity.
( ( (nnn \* 80) +1 ) 250 + mmmm + mmmm - 250 ) / 2
( nnn \* 80 \* 250 + 250 + mmmm \* 2 - 250 ) /2
( nnn \* 80 \* 125 + mmmm )
nnn \*10000 + mmmm
nnn0000 + mmmm
nnnmmmm
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### Author:  [@Rysto](https://boards.straightdope.com/u/Rysto)
#### Post date: [October 12, 2007, 4:11pm UTC](https://boards.straightdope.com/t/amazing-math-forward-to-all-your-friends-and-you-will-be-rewarded/422481/3 "2007-10-12T16:11:03Z")
I assume that it gives your phone number back to you? Let x be the first three digits of your phone number, y be the last four:
1. Grab a calculator (you won’t be able to do this in your head)
I beg to differ…
1. Key in the first three digits of your phone number (not the area code).
We have x
1. Multiply by 80
80x
1. Add 1
80x + 1
1. Multiply by 250.
20000x + 250
1. Add the last 4 digits of your phone number
20000x + y + 250
1. Add the last 4 digits of your phone number again.
20000x + 2y + 250
1. Subtract 250.
20000x + 2y
1. Divide number by 2
10000x + y
Now, 10000x is just x followed by four zeros. Adding y to it will make your phone number.
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### Author:  [@Eureka](https://boards.straightdope.com/u/Eureka)
#### Post date: [October 12, 2007, 4:17pm UTC](https://boards.straightdope.com/t/amazing-math-forward-to-all-your-friends-and-you-will-be-rewarded/422481/4 "2007-10-12T16:17:23Z")
Thank you **rysto** for sparing me from having to type what you did in a slightly different format.
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### Author:  [@brad\_d](https://boards.straightdope.com/u/brad_d)
#### Post date: [October 12, 2007, 4:26pm UTC](https://boards.straightdope.com/t/amazing-math-forward-to-all-your-friends-and-you-will-be-rewarded/422481/5 "2007-10-12T16:26:46Z")
Call the 3-digit prefix of your phone number P, and the 4-digit suffix S.
Write that formula out algebraically:
( (80P + 1)\*250 + S + S - 250 ) / 2
Reduce it to:
10000P + S, which will be your phone number, written out.
(I see others have answered on preview, but I’ll post it anyway. 😉 )
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### Author:  [@tdn](https://boards.straightdope.com/u/tdn)
#### Post date: [October 12, 2007, 4:38pm UTC](https://boards.straightdope.com/t/amazing-math-forward-to-all-your-friends-and-you-will-be-rewarded/422481/6 "2007-10-12T16:38:17Z")
Thanks, peeps! It really is quite simple, I just couldn’t see it.
I’m usually able to figure these things out. I’m still contemplating eating berries with my jackrabbit form Djibouti.
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### Author:  [@Giles](https://boards.straightdope.com/u/Giles)
#### Post date: [October 12, 2007, 5:06pm UTC](https://boards.straightdope.com/t/amazing-math-forward-to-all-your-friends-and-you-will-be-rewarded/422481/7 "2007-10-12T17:06:47Z")
Of course, there’s an assumption that a phone number is 7 digits long. This is true in Zone 1, but not necessarily true in other zones. For example, in Zone 61, local phone numbers have 8 digits, with a two-digit area code.
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### Author:  [@Cemetery\_Savior](https://boards.straightdope.com/u/Cemetery_Savior)
#### Post date: [October 12, 2007, 5:20pm UTC](https://boards.straightdope.com/t/amazing-math-forward-to-all-your-friends-and-you-will-be-rewarded/422481/8 "2007-10-12T17:20:04Z")
[QUOTE=Rysto]
1. Grab a calculator (you won’t be able to do this in your head)
I beg to differ…
[/QUOTE]
Arrogant bastard\*!
-Cem
\*I am duly impressed.
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### Author:  [@Cervaise](https://boards.straightdope.com/u/Cervaise)
#### Post date: [October 12, 2007, 5:30pm UTC](https://boards.straightdope.com/t/amazing-math-forward-to-all-your-friends-and-you-will-be-rewarded/422481/9 "2007-10-12T17:30:27Z")
[QUOTE=Rysto]
Now, 10000x is just x followed by four zeros. Adding y to it will make your phone number.
[/QUOTE]
Beautifully done.
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### Author:  [@Rysto](https://boards.straightdope.com/u/Rysto)
#### Post date: [October 12, 2007, 5:50pm UTC](https://boards.straightdope.com/t/amazing-math-forward-to-all-your-friends-and-you-will-be-rewarded/422481/10 "2007-10-12T17:50:11Z")
[QUOTE=Cemetery Savior]
Arrogant bastard\*!
-Cem
\*I am duly impressed.
[/QUOTE]
Well, it helps that I got to write out every step like that. I would have gotten lost trying to track things in my head.