Comments on: How is this lame math trick possible?
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Comments on Ask MetaFilter post How is this lame math trick possible?Sat, 06 May 2006 12:56:13 -0800Sat, 06 May 2006 12:56:13 -0800en-ushttp://blogs.law.harvard.edu/tech/rss60Question: How is this lame math trick possible?
http://ask.metafilter.com/37683/How-is-this-lame-math-trick-possible
I know this is lame, but how is this lame math trick possible? <br /><br /> 1. Grab a calculator<br>
2. Key in the first three digits of your phone number (NOT the area code)<br>
3. Multiply by 80<br>
4. Add 1<br>
5. Multiply by 250<br>
6. Add the last 4 digits of your phone number<br>
7. Add the last 4 digits of your phone number again<br>
8. Subtract 250 <br>
9. Divide number by 2<br>
<br>
The final sum should be your phone number. How in the hell does this work?!post:ask.metafilter.com,2006:site.37683Sat, 06 May 2006 12:50:18 -0800JPowersmathtrickphone-numberBy: Wet Spot
http://ask.metafilter.com/37683/How-is-this-lame-math-trick-possible#583277
First three digits are x, last four digits are y.<br>
<br>
Your telephone number is 10,000x + y, agreed?<br>
<br>
Ok, take your instructions. You're forming this expression:<br>
<br>
((80x+1)250 + 2y -250)/2<br>
<br>
Simplify:<br>
<br>
(80x+1)125 + y -125<br>
<br>
80*125*x+y<br>
<br>
10000x + y<br>
<br>
There you go!comment:ask.metafilter.com,2006:site.37683-583277Sat, 06 May 2006 12:56:13 -0800Wet SpotBy: Wolfdog
http://ask.metafilter.com/37683/How-is-this-lame-math-trick-possible#583279
Say 'x' for the number formed from the first three digits and 'y' for the number formed from the last three digits (eg, x=555, y=1212).<br>
<br>
Now, your phone number is 10000x+y, agreed?<br>
<br>
What you're calculating looks like this, step by step<br>
<br>
x<br>
80x<br>
80x+1<br>
250(80x+1)<br>
250(80x+1)+y<br>
250(80x+1)+2y<br>
<br>
expanding that just a little, after step 7 you have got<br>
20000x+250+2y<br>
<br>
Now, subtract 250 to get<br>
20000x+2y<br>
and divide by 2 to get<br>
10000x+y,<br>
which we agreed up above is your phone number. Yay.comment:ask.metafilter.com,2006:site.37683-583279Sat, 06 May 2006 12:57:40 -0800WolfdogBy: SeizeTheDay
http://ask.metafilter.com/37683/How-is-this-lame-math-trick-possible#583282
On preview, I got beat by 3 minutes...Curses.comment:ask.metafilter.com,2006:site.37683-583282Sat, 06 May 2006 12:59:56 -0800SeizeTheDayBy: box
http://ask.metafilter.com/37683/How-is-this-lame-math-trick-possible#583283
It works because what you're doing is keying in the first three digits, multiplying by 10000 and then adding the last four. The rest is misdirection.<br>
<br>
(On preview, I might as well remove my clumsy algebra.)comment:ask.metafilter.com,2006:site.37683-583283Sat, 06 May 2006 13:01:01 -0800boxBy: onalark
http://ask.metafilter.com/37683/How-is-this-lame-math-trick-possible#583284
It's an identity.<br>
<br>
Code your phone number as a vector:<br>
<br>
x = [1 2 3 4 5 6 7]<br>
<br>
so x(1) is the first digit, x(2) is the second, etc...<br>
<br>
(*) Then your phone number can be generated from the digits by adding x(7) + 10*x(6) + 100*x(5) ...<br>
<br>
So if we took the first three digits of your phone number y = 250*(80*x[1 2 3] + 1) = 2,000,000*x(1) + 200,000*x(2) + 20,000*x(3) + 250 (this gets us to step 6)<br>
<br>
Then added digits x(6-10) to this, then getting rid of that extra 250 by subtracting it off, then dividing by 2, gets us into the form I mentioned in (*)comment:ask.metafilter.com,2006:site.37683-583284Sat, 06 May 2006 13:01:19 -0800onalark