Math Trick

Here is a math trick so unbelievable that it will stump you.

Personally I would like to know who came up with this. 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 six digits of your mobile number
3. Multiply by 80. Press equal
4. Add 1. press equal
5. Multiply by 250 , press equal
6. Add the last 4 digits of your phone number , press equal.
7. Add the last 4 digits of your phone number again. press equal
8. Subtract 250 , equal
9. Divide number by 2, equal
Do you recognize the answer?