
Setting Up the Trick
Here's one of those mathematical oddities that somewhat masquerades as a magic trick. Your spectator, with a calculator in hand, performs a series of calculations. And surprise, at the end, he or she is looking at their telephone number. We walk you through the steps and at the end, display them all on a single page that you may print out.
Secret: Some mathematical oddity. Ask a math whiz, not a magician.
Ask your spectators to grab their calculators and enter the first three digits of their telephone numbers. Be sure that they enter the first three digits of their actual phone number, not the area code. As an example, we'll use the fictional phone number of 5551212.
Enter 555
Continue to 2 of 5 below. 
Multiply and Add
Ask them to multiply by 80.
555 x 80 = 44400
Ask them to add 1.
44400 + 1 = 44401
Continue to 3 of 5 below. 
Multiply and Add Again
Ask them to multiply by 250.
44401 x 250 = 11100250
Ask them to add the last four digits of their telephone number. And then add the same number again.
11100250 + 1212 = 11101462
11101462 + 1212 = 11102674
Continue to 4 of 5 below. 
Subtract and Divide
Ask them to subtract 250
11102674  250 = 11102424
Ask them to divide by 2. If they've done everything correctly, they'll be looking at their telephone number!
45113 / 2 = 5551212
Here are the steps again:
Enter the first three digits of their phone number
Multiply by 80
Add 1
Multiply by 250
Add the last four digits of their telephone number.
Add the last four digits of their telephone number again.
Subtract 250
Divide by 2Continue to 5 of 5 below. 
The Full Trick
Here are the steps in full again:
 Enter the first three digits of their phone number
 Multiply by 80
 Add 1
 Multiply by 250
 Add the last four digits of their telephone number.
 Add the last four digits of their telephone number again.
 Subtract 250
 Divide by 2