a math trick

Here is a maths trick so unbelievable that it will
stump you. It is pretty amazing

1.Grab a calculator. (you won't be able to do this one
in your head)
2.Key in the first three digits of your land line
telephone number (NOT the area code)
(For 8 digit Tel. Nos. - key in the first four digits
of your phone number)
3. Multiply by 80
4. Add 1
5. Multiply by 250
6. Add the last 4 digits of your phone number
7. Add the last 4 digits of your phone number again.
8. Subtract 250
9. Divide number by 2
Surprised??