Fun with Mathematics

[UncleJoseph]

Ok, a friend emailed this to me. I get these things all the time.

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?

Somebody tell me how these damn things work. I know it's probably simple, like "multiplying any number by two and then dividing that number by two will give you the number you started with," but these formulas always seem more complicated than that, and I've had Calculus 101 and 102.

[Anguirel]

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

Ok, let's do this... Step 4 and step 8 cancel out. Adding 1 prior to multiplying by 250 is the same as adding 250 after and then subtracting it. So that's a total cancellation.

80*250 is 20,000, divided by 2 is 10,000 -- this moves the first three numbers you typed in over 4 places. Adding in twice the remaining four digits, then dividing by two leaves you with just those digits in the thousands column through the singles column.

I haven't done it, but I'd imagine you end up with your phone number. Does that explain it well enough?

[UncleJoseph]

Yeah, that's basically it...as simple as 1+1-1x1=1.

[lorg]

Assume this is for 'american' style phone numbers xxx-xxx-xxxx? Cause it really doesn't work for our style aaaa-xxxxxx. But if you take the Two first instead of three in 2. it will work just fine :)

[Liniah]

Yeah...didn't work for me. (I have 8 digits.)

[Ikarus7]

I didn't work for me either (7 digit)

[Eliahad]

Um, it should, Ik, it's set up for 7 digit phone numbers.

[Ikarus7]

I tried it several time, double checked my numbers and its still doesn't match.

It work with my mother's phone number but not with mine.

[3278]

Same here, and I wouldn't think I'd have made the same error 12 times.

[UncleJoseph]

I think I figured out why it's working for some and not others. With most calculators of today, you can key in several operations before pressing "=" or the calculate key. If you do this, the calculator will obey all rules of arithmetic, and will carry out multiplication and division before doing any addition or subraction.

If you press the "=" or calculate button after every step, the formula should work fine.

Eg: Test phone number 123-4567

123 x 80 = 9840
9840 + 1 = 9841
9841 x 250 = 2,460,250
2,460,250 + 4567 = 2,464,817
2,464,817 + 4567 = 2,469,384
2,469,384 - 250 = 2,469,134
2,469,134 / 2 = 1234567 -- our test phone number

If you wait until the entire formula is entered before pressing "=" or calculate (example above yields 19,099,) the answer will be slightly off, or way off, depending on when and how many times you used the "=" or calculate buttons.

[Ikarus7]

I tried both my science calculator and the windows calculator with the two you described and it still doesn't match my phone number

[UncleJoseph]

Ik,

It's a sound formula. It should work for any seven digit phone number. I tried it with 32's number and got his phone number at the end. I don't know where the error would be. The first time I tried it, I used the Windows calculator, and it worked fine. Haven't a clue why it doesn't work for you.

[Marius]

It's not really too complicated. Split your phone number: XXX | YYYY


XXX * 80 * 250 (Steps 3 and 5) = XXX * 20,000 = 2 * XXX * 10^4
It doubles the first three numbers, and adds four zeroes to the end of it. Step 4 is currently irrelevant, because +1 before multiplying is merely the addition of another 250 (Step 5), which only affects the digits in the last four spaces which we've added to hold the last four digits.

So now we have 2 * XXX | 0000 + 250 (Step 4). The last four steps are simply adding the YYYY twice ( = 2*XXX | 2*YYYY + 250), getting rid of the 250, and halving the whole thing ( = XXX | YYYY)

And that is why math is boring.