# Amusing puzzle **URL:** \ **Category:** Miscellaneous and Personal Stuff I Must Share\ **Created:** [November 20, 2003, 11:06pm UTC](https://boards.straightdope.com/t/amusing-puzzle/214383 "2003-11-20T23:06:01Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![BarnOwl](https://avatars.discourse-cdn.com/v4/letter/b/e95f7d/32.png) [@BarnOwl](https://boards.straightdope.com/u/BarnOwl)\ **Post date:** [November 20, 2003, 11:06pm UTC](https://boards.straightdope.com/t/amusing-puzzle/214383/1 "2003-11-20T23:06:01Z")
Can you figure this out? 1. Key into the calculator the first 3 digits of your phone number (not yourarea code). 2. Multiply by 80 3. Add 1 4. Multiply by 250 5. Add last 4 digits of your phone number 6. Add last 4 digits of your phone number again 7. Subtract 250 8. Lastly, divide by 2 The end result will be your phone number. If you can figure this out, please explain how the arithmetic works ---
**Author:** ![Francesca](https://avatars.discourse-cdn.com/v4/letter/f/9fc348/32.png) [@Francesca](https://boards.straightdope.com/u/Francesca)\ **Post date:** [November 20, 2003, 11:07pm UTC](https://boards.straightdope.com/t/amusing-puzzle/214383/2 "2003-11-20T23:07:11Z")
Note: this only works with US phone numbers. ---
**Author:** ![BarnOwl](https://avatars.discourse-cdn.com/v4/letter/b/e95f7d/32.png) [@BarnOwl](https://boards.straightdope.com/u/BarnOwl)\ **Post date:** [November 20, 2003, 11:11pm UTC](https://boards.straightdope.com/t/amusing-puzzle/214383/3 "2003-11-20T23:11:19Z")
Ooops! ---
**Author:** ![Algorithm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/algorithm/32/12204_2.png) [@Algorithm](https://boards.straightdope.com/u/Algorithm)\ **Post date:** [November 20, 2003, 11:19pm UTC](https://boards.straightdope.com/t/amusing-puzzle/214383/4 "2003-11-20T23:19:52Z")
x = the first 3 digits of your phone number y = the last 4 digits of your phone number To mathematically construct a 7 digit phone number out of x and y, use the equation 10000x + y. Now, to make sure the puzzle does the same thing: 1. Put x into calculator 2. Multiply by 80 (now we have 80x in calculator) 3. Add 1 (80x + 1) 4. Multiply by 250 (20000x + 250) 5. Add y (20000x + 250 + y) 6. Add y again (20000x + 250 + 2y) 7. Subtract 250 (20000x + 2y) 8. Divide by 2 (10000x + y) Success! Does this answer your question? ---
**Author:** ![BarnOwl](https://avatars.discourse-cdn.com/v4/letter/b/e95f7d/32.png) [@BarnOwl](https://boards.straightdope.com/u/BarnOwl)\ **Post date:** [November 21, 2003, 12:09am UTC](https://boards.straightdope.com/t/amusing-puzzle/214383/5 "2003-11-21T00:09:57Z")
Yes. Thank you, Algorithm. ---
**Author:** ![Teelo](https://avatars.discourse-cdn.com/v4/letter/t/8491ac/32.png) [@Teelo](https://boards.straightdope.com/u/Teelo)\ **Post date:** [November 21, 2003, 1:44am UTC](https://boards.straightdope.com/t/amusing-puzzle/214383/6 "2003-11-21T01:44:26Z")
My Canadian phone number worked! Thats pretty cool though. ---
**Author:** ![Revtim](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/revtim/32/1042_2.png) [@Revtim](https://boards.straightdope.com/u/Revtim)\ **Post date:** [November 21, 2003, 2:12am UTC](https://boards.straightdope.com/t/amusing-puzzle/214383/7 "2003-11-21T02:12:23Z")
This has to be best correlation of username and post ever seen on this board. > [@](#): > > \*Originally posted by Algorithm \* > \*\*x = the first 3 digits of your phone number > y = the last 4 digits of your phone number > > To mathematically construct a 7 digit phone number out of x and y, use the equation 10000x + y. Now, to make sure the puzzle does the same thing: > > 1. Put x into calculator > 2. Multiply by 80 (now we have 80x in calculator) > 3. Add 1 (80x + 1) > 4. Multiply by 250 (20000x + 250) > 5. Add y (20000x + 250 + y) > 6. Add y again (20000x + 250 + 2y) > 7. Subtract 250 (20000x + 2y) > 8. Divide by 2 (10000x + y) > Success! > > Does this answer your question? \*\* ---
**Author:** ![lilbtagna](https://avatars.discourse-cdn.com/v4/letter/l/4491bb/32.png) [@lilbtagna](https://boards.straightdope.com/u/lilbtagna)\ **Post date:** [November 21, 2003, 2:25am UTC](https://boards.straightdope.com/t/amusing-puzzle/214383/8 "2003-11-21T02:25:00Z")
**Revtim** , that’s exactly what I thought as well. This is really neat, BTW. I’ll have to show my friends. ---
**Author:** ![Chimpy](https://avatars.discourse-cdn.com/v4/letter/c/a9adbd/32.png) [@Chimpy](https://boards.straightdope.com/u/Chimpy)\ **Post date:** [November 21, 2003, 11:52am UTC](https://boards.straightdope.com/t/amusing-puzzle/214383/9 "2003-11-21T11:52:50Z")
it worked on a UK phone number ---
**Author:** ![Shade](https://avatars.discourse-cdn.com/v4/letter/s/2bfe46/32.png) [@Shade](https://boards.straightdope.com/u/Shade)\ **Post date:** [November 21, 2003, 12:00pm UTC](https://boards.straightdope.com/t/amusing-puzzle/214383/10 "2003-11-21T12:00:56Z")
Well, it works on any seven digit phone number (uh, though starting with 0 could be a problem.) If you rephrased “the first three digits” as “all but the last four digits” it would work for any phone number over four digits.