hairylunch (hairylunch) wrote,

The rule of 72

Man, my algebra has gotten rusty . . .

I saw this post about the rule of 72 - basically, you can estimate how long it'll take your money to double at a set interest rate by dividing 72 by the rate. So at 3% interest, it'll take approximately 72/3 or 24 years to double.

So, I started playing with the math (x is the number of years and y is the interest rate) . . .

  • (1+y/100)^x = 2

  • x ln(1+y/100) = ln2

  • x = (ln2)/ln(1+y/100)

And that's where I get stuck . . . I can plot it and see that 72/y approximates (ln2)/ln(1+y/100). Maybe that's the way to do it? Plot the graph, realize that it's an inverse curve, and fit it?

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