[Alpha-Omega]

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 
 

[Martingale]

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

[Mitch]

Where are you getting all those factor 10s?

[Borek]

abcdefg = a*10⁷ + b*10⁶ + .

Defintion of positional (decimal) system.

[Mitch]

geesh, when was I suppose to learn that?

[Borek]

Early in HS probably

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

[Mitch]

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