cool math trick

Haha extreme bo-liao-iness i noe the maths lar dun test me lar but very nice to trick ppl

This math trick is really cool. Just try it

1. Grab a calculator (You won't be able to do this one in your head)
2. 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
Do you recognize the answer?