'nother math thingy

[Petra]

Not so sophisticated and clever as Crumb's, but then I'm number dumb.

Got this office email thing - how does it work, guys?

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 recognise the answer?


Actually, if I wasn't busy at work I could probably work out how it's done - given a few dedicated days solving it, or something.

But, as I'm kinda lazy that way, I was hoping y'all could simply just tell me. Please.

[Zikes]

I tried it with both our lines and came up with 74757 and 83769...don't really see the significance in either number

[viscousmemories]

I didn't get an obviously significant result either.

[Petra]

Hmmm. Our phone numbers are 7 numbers long, maybe that's the difference.

I did it, and it came up with my phone number.

[Skep]

Okay, there's a similar thingy that gives your age and this works on the same principle but gives your phone # as the answer. The formula hides the fact that you're multiplying the first 3 digits by 10,000 and simply adding the last 4 digits, which gives your full phone #.

Let's call the 3 digit prefix "x" and the last 4 digits "y".

Following the instructions will give this expression:

(250(80x + 1) + y + y - 250)/2

Multiply out the numerator to give:

(20000x + 250 + 2y - 250)/2

The 250s cancel, leaving:

(20000x + 2y)/2

Divide, leaving:

10000x + y = Your Phone #

[viscousmemories]

My phone number is 7 digits too, but I just did this again and I get an 8 digit result that's not at all like my phone number.

[Skep]

Then you're not doing it right.

[viscousmemories]

It's weird, but I did it twice and got the same obviously wrong 8 digit number both times.

Then I did it twice more, and got the same obviously wrong 6 digit number both times.

[Skep]

Here's how 555-1234 would work:

555 x 80 = 44400

44400 + 1 = 44401

44401 x 250 = 11100250

11100250 + 1234 = 11101484

11101484 + 1234 = 11102718

11102718 - 250 = 11102468

11102468 / 2 = 5551234

[viscousmemories]

Huh, weird. It worked that time.

Oh well, thanks.

[slimshady2357]

Originally Posted by lunachick

Not so sophisticated and clever as Crumb's, but then I'm number dumb.

Got this office email thing - how does it work, guys?

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 recognise the answer?

Skep has nailed it and given a nice example.

I just wanted to add comments on what they are doing.

3. Multiply by 80
4. Add 1
5. Multiply by 250
8. Subtract 250


As Skep mentioned, multiplying by 80 and then 250 is the same as multiplying by 20,000.

And notice that step 4 is reversed by step 8, they've had you add 1 and then multiply by 250, so basically you've added 250, then in step 8 you subtract that back out. It has nothing to do with how this works and everything to do with distraction/confusion.

So we can drop steps 4 and 8 and reduce steps 3 and 5 to "Multiply by 20,000". So now the list looks like:

2. Key in the first three digits of your phone number (NOT the area code)
3. Multiply by 20,000
6. Add the last 4 digits of your phone number
7. Add the last 4 digits of your phone number again.
9. Divide number by 2

Step 9 is to divide by 2, or we can just change one thing in the list and get rid of another. Here is how: Change step 3 to look like "Multiply by 10,000" (since we divide the 20,000 by 2) and steps 6 and 7 can be reduced to just one "Add the last four digits of your phone number". And now we can ditch step 9. So now the list looks like:

2. Key in the first three digits of your phone number (NOT the area code)
3. Multiply by 10,000
6. Add the last 4 digits of your phone number

Not so mysterious now

2. Start with phone number XXX-YYYY

3. XXX times 10,000 = XXX0000
4. XXX0000 + YYYY = XXXYYYY

Adam

[livius drusus]

That. Is. So. Cool. Thank you, Adam. .

[Shake]

There was some other thing going around via email saying that "<insert year> is the only year this will work... " and IIRC, it involved one's birthdate or something, but in the end the 'trick' was something similar to the one in this puzzle.