A lame, but interesting math trick that I found yesterday:

What you’ll need:

• U.S. phone number (area code optional)
• Calculator

Steps

1. Put in the 1st three digits of your number (NOT the area code)
2. Multiply Step 1 by 80
3. Add 1 to Step 2
4. Multiply Step 3 by 250
5. Add the last 4 digits of your number and Step 4
6. Add the last 4 digits of your number and Step 5
7. Subtract 50 from Step 6
8. Divide Step 7 by 2.

And voila, you have back the phone number.