[zaphod4prez]

Sorry if I’m missing something, but… what’s the problem with that quote? That’s a widely-used heuristic that helps to estimate doubling times without using a calculator (see [the Wikipedia entry](https://en.m.wikipedia.org/wiki/Rule_of_72).

He does walk the reader through a lot of “back of the napkin” math, in order to help the reader get an intuitive sense of the models he’s using. But my impression overall is that he backs those hand-wavey calculations up with more serious calculations throughout the book.

[Retric]

The issue is he then uses the approximations to do with math without calling them approximations. 1.10^7 is reasonably close to 2, but 1.1^21 is 7.4 which is a fair distance from 8.

He goes so far as asks someone to do the approximation across several hundred years of compounding. And sure it get’s a big number but one no even close to accurate.

[Loanor]

You do realize the rule of 70 is used to approximate continuous compounding not periodic compounding right?

[Retric]

There are a bunch of them though compounding normally uses the rule of 69 / 69.3, or rule 72. https://en.wikipedia.org/wiki/Rule_of_72