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# What Is “Casting Out Nines”? Part 1

Learn how to use the “casting out nines” method to quickly and easily check your answers to arithmetic problems.

By
Jason Marshall, PhD
5-minute read
Episode #82

## Can Casting Out Nines Be Used to Check Other Things?

While that’s pretty amazing, you’ll be happy to know that casting out nines can be used for more than just addition. In fact, as we’ll find out next week, casting out nines can be used to quickly and easily check your answers to multiplication, subtraction, and division problems, too.

## Number of the Week

This week’s number is 7 billion. Why? Well, as you may have heard, humanity reached a milestone sometime around the end of October 2011. For the first time in Earth’s history 7 billion human hearts simultaneously beat out their songs of life upon its surface.

While that sounds exciting, the reality is a lot more complicated. Around the year 1800, the human population began growing at an alarmingly insane rate. The simple math says that if things didn’t slow down, we’d literally run out of room and resources. But fortunately, it’s not all doom and gloom since the growth rate of the human population has recently been declining, and is expected to continue doing so. That’s good news for the Earth and its inhabitants since you can have too much of a good thing!

## Wrap Up

Okay, that’s all the math we have time for today. In the meantime, remember to become a fan of the Math Dude on Facebook where you’ll find a new featured number or math puzzle posted each and every weekday. And if you’re on Twitter, please follow me there too. Finally, if you have math questions, feel free to send them my way via Facebook, Twitter, or by email at mathdude@quickanddirtytips.com.

Are you often stumped for the perfect way to say or write something? Avoid using the same words over and over with Grammar Girl’s 101 Words to Sound Smart.

Until next time, this is Jason Marshall with The Math Dude’s Quick and Dirty Tips to Make Math Easier. Thanks for reading, math fans!

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Images Coursey of Shuttershock.

## About the Author

Jason Marshall, PhD

Jason Marshall is the author of The Math Dude's Quick and Dirty Guide to Algebra. He provides clear explanations of math terms and principles, and his simple tricks for solving basic algebra problems will have even the most math-phobic person looking forward to working out whatever math problem comes their way.

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