An Extremely Cool Math Trick

How to Not Suck at Math, part 9

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This is part of my series about how to help yourself or your kids improve in mathematics. It’s based partly on my own experience of having to learn how to learn mathematics, partly on extensive tutoring experience, and partly on many conversations with homeschooling parents, as well as parents struggling to understand Common Core mathematics. Many future editions are already planned, but feel free to leave suggestions for future editions in the comments (open for paid subscribers) or by email to hollymathnerd at gmail dot com.

The first five posts are not behind the paywall; all future posts will be; this link will give you 10% off. If you’d like to get them but can’t afford a paid subscription, email me at hollymathnerd at gmail dot com and I’ll give you a free one.

Previous posts in the series:

Part 1: Addition and Subtraction

Part 2: Multiplication, Division, and Fractions

Part 3: The Major Key of Mathematical Fluency

Part 4: A Proof for this Approach to Numeracy

Part 5: “I’m Just Not A Math Person!”

Part 6: How the Sign Rules Work, and Why

Part 7: The Box Method of Multiplication

Part 8: Elite Mathematical Training

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If you ever need or want to know if a number is divisible by by 9, there’s a simple way to find out. Add up the digits. If the answer is divisible by by 9, so is the overall number.

This will work 100% of the time, and it’ll even work for huge numbers where you have to repeat the process — where the sum of the digits is so big that you get a new number whose divisibility you’re unsure of, so you do it again.

This same process will work for 3 and numbers divisible by 3, too, not just 9.

Why does this work? It’s very cool, and also very easy to understand!