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jmfbahciv wrote:
> 
> Jack Fearnley wrote:
> > On Sun, 18 Jun 2017 13:21:35 +0000, jmfbahciv wrote:
> >
> >
> >>>
> >> But it's a fast way to estimate the time to double the original amount
> >> when the interest or dividends are reinvested.
> >>
> >> IIRC, divide 72 by the percentage.  That will give the number of year to
> >> double the original amount.
> >>
> >> /BAH
> >
> > The exact doubling time is log(2)/log(1+r/100).
> > If we expand log in a Taylor series and take only the first term
> > we get the approximate doubling time as 100*log(2)/r.
> > That is 69.315/r.
> >
> > So why don't we use a rule of 69 or a rule of 70?
> >
> > It turns out that, in the range 1% to 10% and rounded to whole years, 72
> > is more accurate than 69.315.
> >
> > I found this quite surprising.
> 
> There are a lot of accounting tricks having to with 9s.  I suppose people
> aren't being taught these rules since most accounting is done by computer
> rather than "by hand".  ISTM there was a book published which described
> these "9" tricks.  I always meant to figure out proofs for them but
> life got in the way :-).  When you first encounter these tricks of the
> trade, you think it's magic.
> 
> /BAH


You mean like the value someone came up with the speed of light accounting trick: 

299,792,458 meters per second


299 



it ends like a 99.9  percent ( how these guys think when they are unsure)


299...just short of 300


keeping it under the true value of over 300,000,000.



Then I notice other "9" patterns...


299,792,458


792

7+2=9

458

4+5=9


299,792,458 meters per second is an accounting trick, isn't it????