04 January 2015

Rule of 72 Revisited - The Maths Behind

As we read about investment, we will eventually come across the "Rule of 72" (see previous post on this: Rule of 72 and Rue of 72). Kind of nifty as a rule of thumb to estimate the effect of compounding.

But what is the maths behind the Rule of 72? Business Insider explains in Why the Rule of 72 Works. This should really be used to bring maths alive in school (A-levels?)!

So in fact, we discover that it ought to have been the "Rule of 69". Hah!

Aside from the reason explained in the article for shifting to 72 (i.e. ease of divisibility by a greater range from 1-10%), I guess 69 isn't preferred either for its own reason!?

As an approximation, we could use 72 for dividing 2, 3, 4, 6, 8 or 9%; and 70 for 5, 7 or 10%.

Have fun!

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