# Internet Email: Magic Telephone Number Problem

I received the following email on President's Day - February 16, 2004

O.K. All you brilliant people, let me know how this one is done...

You'll be surprised by the answer. So make sure you do this. Here is a math trick so unbelievable that it will stump you. Personally, I would like to know who came up with this and why that person is not running the country....

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.

Do you recognize the answer?

Yes. It's my number .... FREAK?

Hint: Compare the doing with the undoing. Doing, like adding and multipling and Undoing, like subtracting and dividing.

Discussion:

1 - With algebra and letters ...

Suppose my phone number was xyz abcd.
If I write it as a seven digit number I would get x yza bcd

So, with that in mind, lets work the problem.

(((((xyz * 80 ) +1 ) * 250 ) + abcd) + abcd) - 250) / 2 simplify with algebra;
because (xyz *( (80 + 1) * 250) + 2(abcd) - 250) / 2 is equal to;
(xyz * (81 * 250) + 2(abcd) - 250) / 2 is equal to;
(xyz * 20 250) + 2(abcd) - 250) / 2 is equal to;
(xyz * 20 250 - 250 ) + 2(abcd) ) / 2 is equal to;
(xyz * 20 000 ) + 2(abcd) ) / 2 is equal to;
((20 000 * xyz) + 2(abcd)) / 2 is equal to;
10 000 * xyz + abcd is equal to;
xyz abcd

2 - With a phone number...

Substitute your phone number for xyz and abcd and work through the equations, that might help.

Suppose the your phone number was 123 4567 or 1 234 567

(((((123 * 80) + 1 ) * 250 ) + 4 567) + 4 567) - 250) / 2 solve
(((((9 840 + 1 ) * 250 ) + 4 567 ) + 4 567 ) - 250) / 2
((((9 841 * 250 ) + 4 567) + 4 567) - 250) / 2
(((2 460 250) + 4 567 * 4 567) - 250 ) / 2
((2 460 250 - 250) + (4 567 + 4 567) / 2
((2 460 250 - 250) + (2 * 4 567) / 2
((2 460 000) + (2 * 4 567) / 2
((2 * 1 230 000) + (2 * 4 567) / 2
1 230 000 + 4 567
1 234 567

Mind boggler: To prove you should be running the country make your own.

Like ...

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 300
4. Add 100
5. Multiply by 100
6. Add the last 4 digits of your phone number
7. Add the last 4 digits of your phone number again.
8. Add the last 4 digits of your phone number one last time
9. Subtract 10 000
10. Divide number by 3

Do you recognize the answer?

Enjoy!