Grab a calculator. (you won't be able to do this one in your head)
  1. Key in the first six digits of your mobile number
  2. Multiply by 80
  3. Add 1
  4. Multiply by 250
  5. Add the last 4 digits of your phone number
  6. Add the last 4 digits of your phone number again.
  7. Subtract 250
  8. Divide number by 2
  9. Do you recognize the answer?