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Econ 101 of the Day, 1/10/2014

Since it’s now spring semester (my spring semester starts very early, so I am jealous of all of you who are still on break), I figured it would make sense to shift over to macroeconomics a bit rather than exclusively focusing on microeconomics. (It also doesn’t hurt that I’m teaching graduate macro this semester and want to give my students some review materials.) MY students are currently getting into economic growth models, so I figured it would make sense to write up a refresher on some growth math:

Gotta love that mathematical precision gives way to the fact that students giggle too much at a “rule of 69.” 🙂 (And no, I can’t decide if I want to be the pot or the kettle.) See here for an overview of classroom-type materials available on the site.

**Tags:** Econ 101 of the Day

## 1 response so far ↓

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Dave Tufte// Jan 19, 2014 at 2:22 pmDon’t do rule of 70 … do rule of 72.

The ancient Mesopotamians (I’m not sure which culture) were on to something when they based their big arithmetic thinking on numbers that could be factored into just small prime numbers.

So, your classroom examples will work better with 72 because it’s divisible by 2, 3, 4, 6, 8, 9, 12, 18, 24, and 36. The extra accuracy you might get from using 70 instead of 72 is lost when the students can’t do the division in their heads.

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