I did the same thing as xxxphoto but wrote out all the steps. The steps are to the right of the calculation. It's easy to follow. Peaches?
xxx is the first 3 digits of phone number
yyyy is the last 4 digits of phone number
1. xxx Key into the calculator the first 3 digits of your phone number
2. 80xxx Multiply by 80
3. 80xxx+1 Add 1
4. 250*(80xxx+1) Multiply by 250
5. 250*(80xxx+1)+yyyy Add last 4 digits of your phone number
6. 250*(80xxx+1)+2yyyy Add last 4 digits of your phone number AGAIN
250*80xxx + 250 + 2yyyy (simple step. multiply out the 250 now for bookkeeping as we know we need to subtract that 250 next step)
7. 250*80xxx+2yyyy Subtract 250
8. 125*80xxx+yyyy Lastly, divide by 2
=10,000xxx+yyyy (Then just multiply the 125 and the 80 so you can see what the answer looks like)
And of course 10,000xxx+yyyy is your phone number. You need the 10,000 as a placeholder just like 27 is 10*2+7.
8675309 is really 10,000*867+5309.
This trick wouldn't work in ancient Rome. They didn't have a base 10 placeholding number system with a zero. You can probably thank the Arabs.
Last edited by Colin at Feb 10 2004, 06:27 AM
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