1. Key in the first three digits of your seven-digit phone number into the calculator. (Note: Do not key in the area code, but the three digits after.)
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 nmber again.
7. Subtract 250.
8. Divide by 2.
Amazing, right? I love this puzzle. I emailed my solution/explanation to this puzzle.
This can also work for cellphone numbers if you use only the last seven digits (ie d not include 0916, 0917, 0918, 0920, 0906, or whatever the first four digits of your mobile number is) of your mobile number.