[Aeronaut]

I can't figure it out, but it did work.

[PetroniDE]

I will explain soon.... It did work.... I think I know why.

[thoughtomator]

First 3 digits = X; last 4 = Y
Phone number = 10,000X + Y

(250*(80X+1) + 2Y - 250)/2 is then the expression of the steps indicated.

So our equation is
(250(80X+1)+ 2Y -250)/2 = 10,000X +Y

factoring this out brings us to
(20,000X + 250 + 2Y -250)/2 = 10,000X +Y

drop the 250s as they cancel each other out, and do the division:

10,000X + Y = 10,000X + Y


(I learned to do this math in 4th grade, if I remember correctly)

[4mycountry]

Cool! :)

[PetroniDE]

IT DOES WORK.... HERE'S WHY.

Step 1 is the first input. Call it "X".

Steps 2 through 4 result in the following. "Z" is the running total on your calculator.
Z = 20000 (X) + 250
The 20000 once divided by 2, simply adds five zeros to the phone number prefix and reserve the spaces for the phone number suffix.

Steps 5 and 6 simple added twice the suffix. Call the phone number suffix "Y".

Step 7 simply negates the 250 from the formula for Z above. This is window dressing.

Step 8 divides the total by 2. The 20000 becomes 10000 as explained above and the second suffix addition is eliminated (resulting in one net suffix addition).

[elbucko]

I'd be more impressed if the problem did not ask one to enter ones complete phone number. The rest of the "problem" is just, as one poster claimed, "window dressing", fooling you into thinking that the correct number that was produced by the "formula" was esoteric in origin.

[Khurkris]

It works for me.

[LiteKeeper]

bump

[EuroFrog]

It didnt work for mine. Then again, my number is only 6 digits. All the numbers were there with a zero in the middle of it.