Fun with math

[tubaman5150]

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?

[Tubaryan12]

cool

[Thomas Maurice Booth]

What's it supposed to come out to...I got nothing recognizable after several attempts...

[Tubaryan12]

Hey, Fairfield! If done correctly the answer should be your phone number (without area code, of course)

[Chuck(G)]

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?

I recognize some not-very clever fiddling with numbers. Let's see:

Steps 4 and 5 add 250 to the intermediate result and 8 removes 250, so you can just drop steps 4 and 8.

6 and 7 add double the last 4 digits of the your phone number and 9 halves it and the 80 you multiply by in 3. So change the 80 to 40 in 3 and drop steps 7 and 9.

So we have:

1. Grab a calculator
2. Enter the first 3 digits of your phone number
3. Multiply by 40
5. Multiply by 250
6. Add the last 4 digits of your phone number

But 40 x 250 is 10,000, so we can reduce this to:

1. Grab a calculator
2. Enter the first 3 digits of your phone number
3. Multiply by 10,000
6. Add the last 4 digits of your phone number

Or, recognizing that multiplying by 10,000 shifts any number 4 places to the left, we can reduce this to:

1. Grab a calculator
2. Enter your phone number

Do you recognize the answer?

(Note that no equations have been used here, just plain old 3rd grade arithmetic.).

[Chuck(G)]

Or, recognizing that multiplying by 10,000 shifts any number 4 places to the left, we can reduce this to:

1. Grab a calculator
2. Enter your phone number

Do you recognize the answer?

(Note that no equations have been used here, just plain old 3rd grade arithmetic.).

where's the fun in that?

Hey, you don't know some of the folks who frequent this forum!

[Rick Denney]

Mmmm. Do-nuts.

Rick "yawn" Denney

[windshieldbug]

All I can get is

[bort]

Very true, nice deduction Chuck. Many simliar problems like this one really only come down to a few simple steps or rules involving number theory. Even problems where someone is able to "guess" your number are actually pretty simple if you can understand things like various numerical properties and binary numbers.

Btw. I did actually do the whole problem out before I discovered Chuck's post and was about to try and figure out the secret. But after I read the following posts I realized Chuck beat me to it.

Ditto - I write math books for a living, so I was looking forward to figuring out the gimmick. Too bad I read everything first! The others are right - there's a million tricks like this out there; it's a matter of making it seem like you're doing something more complicated.