Try this puzzle...need a caculator

1) Key-in the first 3 digits of your phone number into the calculator
(not the area code)
2) Multiply by 80
3) Add 1
4) Multiply by 250
5) Plus last four digit of phone number
6) Plus last four digit of phone number again
7) Minus 250
8- Divide by 2

Is it your phone number?

The 250 is just a confusing issue, made to make this look moer mystical than it is...
The first four instructions...

you start with your phone number. Since it's a total of 7 digits and the length of the first three (the exchange) is three, we need to offset the exchange by 4 places, or 10,000. If you look at instructions 2 through 4, you can see we're multiplying by 20,000 (80x250) (and adding 250).
(try this, 123 x 80 =9840 + 1=9841, x250 = 2460500).

As you can see, if your phone number started with exchange 123 we've shifted them to their proper place, and have an extra 250, all doubled.

The final instructions
So then they have you add the last four digits of your number twice, subtract that silly 250, and have you divide by 2.

OK so here it is wihtout the extra 250 stuff
enter the three digit exchange
multiply by 20,000
add in the last four digits of your phone number doubled
divide by 2.

So let's simplify this just a little bit further, by removing the need to divide by 2.
enter the three digit exchange
multiply by 10,000
add in the last four digits of your phone number

I get frustrated by these things, because they just are designed to confuse people.