Puzzling Week: Number Riddles

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 phone number (NOT the area code).
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

Does the answer look familiar?
Algebraically, why does this work?
Make up your own number riddle and explain algebraically why it works.

My thanks to lynnemead07!

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