I had to look it up at wikipedia, but apparently, 72 comes from the
fact that it is easier to divide integers into 72 than 70 or 69, which
aids mental calculations. This is because of the factors of each
number: 72=2*2*2*3*3, 70=2*5*7, 69=3*23.

Sean


On Thu, Jul 16, 2009 at 1:43 PM, Marcel
Birthelmer<marcelb.lists@gmail.com> wrote:
>> The "method" uses the classic "rule of 72" - here modified to 'rule of 70'
>> :-).
>
> Maybe I'm thinking of a different "rule of 70", the one used to
> calculate how long it will take for a given amount of initial
> investment to double at a fixed interest rate? In that case, it's the
> rule of 70 because ln(2) = .69... , so I'm not sure where the Rule of
> 72 is coming from here.
> - Marcel
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