Re: The Rule Of 72

From	jmfbahciv <See.above@aol.com>
Newsgroups	sci.math, sci.physics, alt.astronomy
Subject	Re: The Rule Of 72
Date	2017-06-19 12:43 +0000
Organization	"Have EDDT, will travel"
Message-ID	<PM0005524F97315FF5@aca4299c.ipt.aol.com> (permalink)
References	(15 earlier) <59441500.660A@ix.netcom.com> <Pine.NEB.4.64.1706180046100.4820@panix5.panix.com> <oi5q6q$1oip$1@gioia.aioe.org> <PM0005523BFFEC5FCC@aca43d4d.ipt.aol.com> <oi6hiq$2p5$1@dont-email.me>

Cross-posted to 3 groups.

Show all headers | View raw

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

Thread

The Rule Of 72 The Starmaker <starmaker@ix.netcom.com> - 2017-06-16 10:27 -0700
  Re: The Rule Of 72 Dan Christensen <Dan_Christensen@sympatico.ca> - 2017-06-16 11:05 -0700
  Re: The Rule Of 72 William Elliot <marsh@panix.com> - 2017-06-18 00:46 -0700
    Re: The Rule Of 72 Sегgi о <iinvalid@invalid.com> - 2017-06-18 07:03 -0500
      Re: The Rule Of 72 jmfbahciv <See.above@aol.com> - 2017-06-18 13:21 +0000
        Re: The Rule Of 72 Jack Fearnley <jack@alcor.concordia.ca> - 2017-06-18 18:42 +0000
          Re: The Rule Of 72 Sегgi о <iinvalid@invalid.com> - 2017-06-18 14:18 -0500
            Re: The Rule Of 72 jmfbahciv <See.above@aol.com> - 2017-06-19 12:43 +0000
              Re: The Rule Of 72 Sегgi о <iinvalid@invalid.com> - 2017-06-19 08:45 -0500
                Re: The Rule Of 72 jmfbahciv <See.above@aol.com> - 2017-06-20 12:12 +0000
          Re: The Rule Of 72 jmfbahciv <See.above@aol.com> - 2017-06-19 12:43 +0000
            Re: The Rule Of 72 The Starmaker <starmaker@ix.netcom.com> - 2017-06-19 10:46 -0700
              Re: The Rule Of 72 The Starmaker <starmaker@ix.netcom.com> - 2017-06-19 20:01 -0700
              Re: The Rule Of 72 jmfbahciv <See.above@aol.com> - 2017-06-20 12:12 +0000
                Re: The Rule Of 72 The Starmaker <starmaker@ix.netcom.com> - 2017-06-20 07:28 -0700
                Re: The Rule Of 72 jmfbahciv <See.above@aol.com> - 2017-06-21 12:54 +0000
                Re: The Rule Of 72 The Starmaker <starmaker@ix.netcom.com> - 2017-06-21 12:19 -0700
                Re: The Rule Of 72 jmfbahciv <See.above@aol.com> - 2017-06-22 12:28 +0000
                Re: The Rule Of 72 The Starmaker <starmaker@ix.netcom.com> - 2017-06-22 08:58 -0700
                Re: The Rule Of 72 jmfbahciv <See.above@aol.com> - 2017-06-23 11:40 +0000
                Re: The Rule Of 72 The Starmaker <starmaker@ix.netcom.com> - 2017-06-23 10:11 -0700
                Re: The Rule Of 72 jmfbahciv <See.above@aol.com> - 2017-06-24 11:40 +0000
                Re: The Rule Of 72 The Starmaker <starmaker@ix.netcom.com> - 2017-06-24 09:12 -0700
                Re: The Rule Of 72 jmfbahciv <See.above@aol.com> - 2017-06-25 13:50 +0000
                Re: The Rule Of 72 Jack Fearnley <jack@alcor.concordia.ca> - 2017-06-20 14:54 +0000
                Re: The Rule Of 72 jmfbahciv <See.above@aol.com> - 2017-06-21 12:54 +0000
    Re: The Rule Of 72 djheydt@kithrup.com (Dorothy J Heydt) - 2017-06-18 14:07 +0000
      Re: The Rule Of 72 David Bernier <david250@videotron.ca> - 2017-06-18 12:38 -0400
        Re: The Rule Of 72 djheydt@kithrup.com (Dorothy J Heydt) - 2017-06-18 19:27 +0000
    Re: The Rule Of 72 "hanson" <hanson@quick.net> - 2017-06-18 16:07 -0700

csiph-web