Posted: 4/24/2005 7:42:35 PM EDT
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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 |
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Explained: (ETA:Copied from a Usenet post; I am no math geek) The problem is pretty simple. We start with the left-most three digits of a 7-digit number. The first thing we must do is get them into their correct position relative to the right-most four digits which will come in later. In simple terms, this is done by adding 4 zeros to the end of the number (multiplying by 10,000). UrTrouble’s trick does this in multiple steps to conceal the simplicity. He multiplies by 80, adds 1, and then multiplies by 250. What has this done? Basically it has put the first three numbers in their correct relative position, doubled them, and added an extraneous 250. See below: ((### x 80) + 1) x 250 = (### x 80 x 250) + 250 = (### x 20,000) + 250 The next step is to insert the right-most four digits. This is done simply by adding them, and then adding them again. Why twice? Well remember, the original number is in the correct relative position and doubled, so adding the last four digits twice makes the whole number doubled- but now we’re stuck with that extraneous 250. Subtracting 250 easily handles that problem. Then all we need to do is divide by two and VIOLA!, our original number. Below is a simple synopsis of what the trick does step by step LLL -> L,LL0,000 x 2 + 250 -> L,LLR,RRR x 2 + 250 -> L,LLR,RRR x 2 -> L,LLR,RRR I must admit, when I first saw this problem yesterday, I considered it a possibility that phone numbers were derived by some obscure algorithm (Ma Bell loved that kind of stuff). If that were true then the first three numbers (prefix) of every phone number would have to conform to some rigid relationship with the last four digits. Obviously, this is not the case at all. Any seven digit number will yield the expected results when using the ‘math trick’ above. Thanks again UrTrouble, good stuff! |
I must be retarded cause I'd never have figured this out. Btw, the LLL-> and all that dont mean a damned thing to me.... anyhew it worked for me and I'm happy to leave it a mystery. |
