Phone number math...

[DrDoc]

Fire up the calculator...
Type in the first three digits of your phone number (not area code)...
...multiply by 80
...add 1
...multiply by 250
...add the last 4 digits of your phone number
...add the last 4 digits again
...subtract 250
...divide by 2

Recognize the number? ;)

Note: This only works with US/Canada style phone numbers...

[Chndru]

ABC-DEFG = A*1,000,000+B*100,000+C*10,000+D*1,000+E*100+F*10+G

ABC = A*100+B*10+C*1 
DEFG = D*1000+E*100+F*10+G*1

((((ABC)*80+1)*250+2*(DEFG))-250)/2

= (((((A*100+B*10+C*1)*80+1)*250+2*(D*1000+E*100+F*10+G*1))-250)/2

= ((((A*8000+B*800+C*80+1)*250+(D*2000+E*200+F*20+G*2))-250)/2

= ((A*2000000+B*200000+C*20000+250+D*2000+E*200+F*20+G*2)-250)/2

= (A*2000000+B*200000+C*20000+D*2000+E*200+F*20+G*2)/2

= A*1,000,000+B*100,000+C*10,000+D*1,000+E*100+F*10+G

= ABC-DEFG

it's friday ;)

[volatilegx]

Very nice, and very silly :)

[encyclo]

OK, so you get your full number ;) Of course, it's just an algebra trick - you're multiplying the first 3 figures by 10000 to move them left, then adding the last four digits:

So, if a is the first three digits, b the last four and x the full number, you change the steps into an equation:

x = {(80a + 1)250 + 2b - 250}/2

You can them simplify down the equation:

x = (20000a + 250 + 2b - 250)/2

x = (20000a + 2b)/2

x = 10000a + b

Quod erat demonstrandum. Or something like that...!

Thanks to DrDoc for getting my very rusty mathematics skills back into action!

<added>Beaten to it again! Chndru's explanation was much cooler than mine ;)</added>

[grandpa]

My calculator's broken. I don't get anything like I expect.

[Macguru]

What a great bunch of joyful geniuses, we have here. I feel honored to hang around. Now I will just grab the trick and take all the credits for myself. :)

[brotherhood of LAN]

does this work with credit cards too :)

[DrDoc]

does this work with credit cards too

Sure it does... you just need to change the initial "multiply by 80" to "multiply by 80,000,000,000" :o Of course, then you just add the last 13 digits (instead of four) :)

[ShootinBlanks]

somebody has WAY too much free time... :o)

[geexter]

I did the math using Window's Calculator and came up with the correct number.

I then switched the Window's Calculator to the scientific function and tried the math again.

While using the scientific function, I would continually receive the wrong answer.

Could someone proof this and write back? Thanks.

[DrDoc]

Welcome to Webmaster World, geexter!

You sure you were in decimal mode?

[geexter]

Yep. Decimal is checked.

I just tried my work phone w/ scientific and it came out incorrect.

<snip>

[edited by: lawman at 7:53 pm (utc) on May 27, 2004]
[edit reason] No phone numbers please [/edit]

[DrDoc]

After each calculation, make sure you hit the [=], or else you can get some funky results...

[ronin]

Take the calculator.
Type in the number 5318008.
Turn the calculator upside down.

What does it say?

Phone number math, I ask you...

[snowman]

Cute! :)

[Leosghost]

as per ronin ..
type 379009..
turn upside down ..
the "plex" ownzya

[whoisgregg]

Turn the calculator upside down

You won't imagine the looks I got when I turned my monitor upside down... ;)

[Chndru]

lol, whoisgregg.

[vkaryl]

*snicker* Leo's is cuter than ronin's - from the female angle of course....