# Weird formula that yields a familiar number
**URL:** https://boards.straightdope.com/t/weird-formula-that-yields-a-familiar-number/450837
**Category:** Miscellaneous and Personal Stuff I Must Share
**Created:** [May 28, 2008, 1:16am UTC](https://boards.straightdope.com/t/weird-formula-that-yields-a-familiar-number/450837 "2008-05-28T01:16:06Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@Elendil\_s\_Heir](https://boards.straightdope.com/u/Elendil_s_Heir)
#### Post date: [May 28, 2008, 1:16am UTC](https://boards.straightdope.com/t/weird-formula-that-yields-a-familiar-number/450837/1 "2008-05-28T01:16:06Z")
Just got this via email. Can anyone explain how this works?
Here’s a math trick so unbelievable it will stump you. Grab a calculator (you won’t be able to do this one in your head.)
1. Key in the first three digits of your phone number (NOT the area code).
2. Multiply by 80.
3. Add 1.
4. Multiply by 250.
5. Add the last four digits of your phone number.
6. Add the last four digits of your phone number again.
7. Subtract 250.
8. Divide by 2.
And the answer is…
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### Author:  [@Sierra\_Indigo](https://boards.straightdope.com/u/Sierra_Indigo)
#### Post date: [May 28, 2008, 1:21am UTC](https://boards.straightdope.com/t/weird-formula-that-yields-a-familiar-number/450837/2 "2008-05-28T01:21:53Z")
Not my phone number - there’s a digit missing\*
\*We have differently formatted phone numbers over here. But it does work for Aussiedopers if you change the first step to the first FOUR digits of your phone number 😃
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### Author:  [@Rysto](https://boards.straightdope.com/u/Rysto)
#### Post date: [May 28, 2008, 1:27am UTC](https://boards.straightdope.com/t/weird-formula-that-yields-a-familiar-number/450837/3 "2008-05-28T01:27:04Z")
We’ve done this before. I posted the mathematics behind it – it’s not very hard to see how this trick works.
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### Author:  [@Hirka\_T\_Bawa](https://boards.straightdope.com/u/Hirka_T_Bawa)
#### Post date: [May 28, 2008, 3:07am UTC](https://boards.straightdope.com/t/weird-formula-that-yields-a-familiar-number/450837/4 "2008-05-28T03:07:34Z")
For those interested (I know I was), [here](http://boards.straightdope.com/sdmb/showthread.php?t=439972) is that previous post **Rysto** referred to.
By the way **Rysto** , great explanation in that thread, thanks.
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### Author:  [@Elendil\_s\_Heir](https://boards.straightdope.com/u/Elendil_s_Heir)
#### Post date: [May 28, 2008, 5:11am UTC](https://boards.straightdope.com/t/weird-formula-that-yields-a-familiar-number/450837/5 "2008-05-28T05:11:43Z")
Thanks. I searched the SDMB for the phrase “area code” and it didn’t come up.