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I've been studying the rule of 72 lately, which gives the time in years that it takes for an investment to double based on the annual percentage rate of growth.I have tried to compare it's formula t=72/x (x being the number in front of the percent sign) with the exact value for the doubling time (in years) of an investment based on interest. The exact formula is t=(ln2)/ln(1+(x/100)). This is also where x is the number in front of the % sign. What I don't yet understand is how 72/x is connected to the exact value formula. Iv'e seen the graphs and understand how close they are... but why are they so close?