>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.

I understood that the 72 came from the number of repayments that were made, 
and if these were made monthly, then 72 months was the period of the loan, 
probably typical for a loan for purchasing a home appliance. I believe there 
were ready reckoner books with tables for determining the amount still owed 
if people wanted to repay the loan early, that were calculated using this 
formula, which was how it came into such wide use.

In actual practice the number 72 gets modified for the actual number of 
repayments (e.g. monthly repayments on a mortgage), but the formula is still 
known as 'rule of 72' because of its origins. 

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