Scientific American presents Math Dude by Quick & Dirty Tips. Scientific American and Quick & Dirty Tips are both Macmillan companies.

Long time math fans may remember our first foray into the world of percentages way back in the 12th and 13th episodes of the podcast. In those shows we learned what percentages are, how they’re related to fractions, how to use percentages to easily calculate tips at restaurants, and how to use percentages to easily calculate sales prices when shopping.

If you’re not sure how to perform any of those handy calculations, or if you’re just in need of a general percentage refresher, I highly recommend taking a look at those earlier shows and getting yourself up to speed. Why? Because once you’re caught up, you’ll be ready to step up and learn how to become a true percentage-calculating machine. Which is exactly what we’re going to turn you into today.

Recap: What Are Percentages?

To make sure we’re all on the same page, let’s kick things off by taking a minute to recap a few key facts about percentages. Let’s start with the most important question: What are percentages? Perhaps the most illuminating thing to know is that the word “percent” is really just the phrase “per cent” squashed together. And since “cent” here means 100 (as in “century”), we see that the word “percent” just means “per 100.” In other words, 10% means “10 per 100,” which is the same as the fraction 10/100 or 1/10.

This turns out to be great news since it makes lots of percentages easy to calculate. In particular, it’s easy to calculate 10% of any number since that’s just 1/10 of the number. Why is that so helpful? Because it means that you can quickly calculate 10% of a number simply by moving its decimal point 1 position to the left.

But what about calculating something like 36% of 25? Or maybe 250% of 20? In these cases, our trick of using the power of 10% doesn’t help—so what can we do?

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