How does this work?
According to this, 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
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:
- Grab a calculator (you won’t be able to do this in your head)
I beg to differ…
- Key in the first three digits of your phone number (not the area code).
We have x
- Multiply by 80
80x
- Add 1
80x + 1
- Multiply by 250.
20000x + 250
- Add the last 4 digits of your phone number
20000x + y + 250
- Add the last 4 digits of your phone number again.
20000x + 2y + 250
- Subtract 250.
20000x + 2y
- Divide number by 2
10000x + y
Now, 10000x is just x followed by four zeros. Adding y to it will make your phone number.
Thank you rysto for sparing me from having to type what you did in a slightly different format.
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. 
)
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.
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.
Arrogant bastard*!
-Cem
*I am duly impressed.
Beautifully done.
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.