Americans only

by Volker Weber

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?

[Thanks, Wolfgang]

Resolution (short):

The first steps multiply by 250 and 80 which is 20,000. This shifts the first three digits four times to the left and doubles them. To confuse you, you have to add 1 after the first multiplication which becomes 250. Keep that in mind. Then we add the last four digits twice. We now subtract the 250, so we essentially have double your phone number. Divide that by 2. You have your phone number.

Resolution (long):

A seven digit number "abcdefg" is "a" million, "b" hundredthousand ... So it has a value of 1,000,000 a + 100,000 b + 10,000 c + 1,000 d + 100 e + 10 f + 1 g

## Step 2: Key in the first three digits of your phone number

abc has a value of 100 a + 10 b + c

## Step 3: Multiply by 80

80 * (100 a + 10 b + c) =
8,000 a + 800 b + 80 c

## Step 4: Add 1

8,000 a + 800 b + 80 c + 1

## Step 5: Multiply by 250

250 * (8,000 a + 800 b + 80 c + 1) =
2,000,000 a + 200,000 b + 20,000 c + 250

## Step 6: Add the last four digits

defg has a value of 1000 d + 100 e + 10 f + g

lets add that and we get

2,000,000 a + 200,000 b + 20,000 c + 250 + 1,000 d + 100 e + 10 f + g =
2,000,000 a + 200,000 b + 20,000 c + 1,000 d + 100 e + 10 f + g + 250

** Step 7: Add that again

2,000,000 a + 200,000 b + 20,000 c + 2,000 d + 200 e + 20 f + 2 g + 250

** Step 8: Subtract 250
2,000,000 a + 200,000 b + 20,000 c + 2,000 d + 200 e + 20 f + 2 g + 250 - 250 =
2,000,000 a + 200,000 b + 20,000 c + 2,000 d + 200 e + 20 f + 2 g

** Step 9: Divide by 2

1,000,000 a + 100,000 b + 10,000 c + 1,000 d + 100 e + 10 f + 1 g

which is our seven digit number "abcdefg"

Comments

And now someone say math can't be fun...

Ragnar Schierholz, 2004-02-05

Nice idea. But, of course, it doesn't work very well with a five digit phone number. ;-)

Volker Berding, 2004-02-05

After 3 hours of thinking i've worked this out for 5digit phone numbers: ;-)

1. Grab a blank sheet checkerd paper.
2. Write down the first two digits of your phone number (NOT the area code).
3. Multiply by 40.
4. Add 1.
5. Multiply by 75.
6. Add the last 3 digits of your phone number.
7. Add the last 3 digits of your phone number again.
8. Add the last 3 digits of your phone number again.
9. Subtract 75.
A. Divide number by 3.
B. Be surprised.

Fred Spinner, 2004-02-05

Finally, I am as surprised as promised. Thanks!
Hope you don't mind if I used a caculator instead of a sheet of paper. ;-)

Volker Berding, 2004-02-05

I wonder if "Americans only" and "you won't be able to do this in your head" was an intended juxtaposition. ;-)

Jerry Carter, 2004-02-05

Jerry, no. I was just trying to make sure you have a 7-digit phone#. These riddles are quite easy to construct. You just need to know how many digits the number has. And it has to fit in a calculator.

Volker Weber, 2004-02-05

Ugh... these things are so lame. "Take the first three digits of your phone number, multiply by 10000, add the last four digits. Isn't that AMAZING!?"

No. It's not. Now, show me an algorithm that takes my phone number, performs some calculations on it, and it turns in to my name in base36. *That* would be exciting. :)

Nathan T. Freeman, 2004-02-05

I think it would only be cool if, regardless of the number you enter, the answer would be 42.

Stefan Tilkov, 2004-02-05

It would also be cool if I remembered my grammar lessons.

Stefan Tilkov, 2004-02-05

wow, that failed, it doesn't work for my #, i didn't do my area code... i did 1-***-946-*/*/ i did 946 X 80 + 1 X 250 + */*/ + */*/ - 250 / 2.. */*/=last 4 digits the final result, was 5 digits, that resmbled no part of my phone #...

brendan gaulin, 2005-10-18

never mind, sorry for my 2 posts, my calculator, was on sped mode

Brendan Gaulinn, 2005-10-18

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