[geoschmo]

Originally posted by Jack Simth:

quote:

---

Originally posted by Wardad:
1) key-in the first 3 digit of your phone number into the calculator
2) multiply by 80
3) then plus 1
4) multiply by 250
5) plus Last four digit of phone number
6) plus Last four digit of phone number again
7) minus 250
8) divide by 2 at Last

Is the answer your phone number???

This will leave any positive integer x completely intact:
1) x div 10000
2) (1) * 80 = (x div 10000)*80
3) (2) + 1 = ((x div 10000)*80) + 1
4) (3) * 250 = (((x div 10000)*80) + 1) * 250 = 250*((x div 10000)*80) + 250 = 250*80*(x div 10000) + 250 = 20000*(x div 10000) + 250
5) (4) + x mod 10000 = 20000*(x div 10000) + 250 + (x mod 10000)
6) (5) + x mod 10000 = 20000*(x div 10000) + 250 + 2*(x mod 10000) = 2*10000*(x div 10000) + 2*(x mod 10000) + 250 = 2*10000*(x/10000) + 250 = 2*x + 250
7) (6) - 250 = 2*x + 250 - 250 = 2*x
8) (7) / 2 = 2*x/2 = x

It doesn't matter what number you start with....

---

Here's an explanation of how this works for those of us less mathematically inclined then Jack Simth:

The idea is to take the first three digits of the phone number and add four zeros after it. You do this by multiplying by ten thousand. Then add the Last four digits to that and the result is your phone number. However doing these two simple steps would be easily recognizable and not illicate the proper "Spooky!" reaction, so the rest of the instuctions are combinations of steps that cancel each other out and so have no net effect.

Steps 2, 4, and 8 multiply the first three digits given in step 1 by 80, then by 250, and then divide by 2. This extra complication serves only to confuse and can be simplified down to multiplication by 10,000. (80 x 250 = 20,000, 20,000/2 = 10,000)

Step 3, adding 1, is usless complication meant only to confuse, and is canceled out by step 7, subtracting 250. You have to subtract 250 because in between you multipled by 250 in step 4.

Steps 5 has to be duplicated with step 6 because you are dividing by 2 in step 8.

Pretty neat, but very simple when you see what's happening. It the mathematical equivalent of a magician distracting you with irrelevant movements of his right hand while he pulls a card out of his left pocket.

[ October 20, 2003, 17:25: Message edited by: geoschmo ]