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General Forums => Generic Discussion => Topic started by: Alpha-Omega on February 23, 2008, 01:38:17 AM

Title: Math Trick Phone Number
Post by: Alpha-Omega on February 23, 2008, 01:38:17 AM

Math Trick Phone Number

Here is a math trick so unbelievable that it will stump you. Personally I
would like to know who came up with this and why that person is not running
the country.

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  ;D
 

Title: Re: Math Trick Phone Number
Post by: Martingale on February 23, 2008, 02:29:02 PM

If your phone number is abcdefg then

[((a*100+b*10+c)*80+1)*250+2*(d*1000+e*100+f*10+g)-250]/2

(((a*100+b*10+c)*80+1)*125+(d*1000+e*100+f*10+g)-125)

=(a*100+b*10+c)*10000+125+d*1000+e*100+f*10+g-125

= a*1000000+b*100000+c*10000+d*1000+e*100+f*10+g

=abcdefg

Title: Re: Math Trick Phone Number
Post by: Mitch on February 23, 2008, 06:40:00 PM

Where are you getting all those factor 10s?

Title: Re: Math Trick Phone Number
Post by: Borek on February 23, 2008, 07:25:56 PM

abcdefg = a*10⁷ + b*10⁶ + .

Defintion of positional (decimal) system.

Title: Re: Math Trick Phone Number
Post by: Mitch on February 23, 2008, 07:31:40 PM

geesh, when was I suppose to learn that?

Title: Re: Math Trick Phone Number
Post by: Borek on February 23, 2008, 08:14:44 PM

Early in HS probably :)

http://en.wikipedia.org/wiki/Positional_system

Title: Re: Math Trick Phone Number
Post by: Mitch on February 23, 2008, 09:42:12 PM

Jeebus, what the hell are they not teaching in the californian public education system. Seriously, never seen it in any of my studies.