Written by Elizabeth Hand
Elizabeth is the Digital Media Producer at Science World. She has an MFA in Creative Writing and loves to write. She writes blogs, writes code and writes science fiction. Follow the updates @bettyHand, @scienceworldca, and @scienceworldtr


Math Trick

Created date

Friday, November 3, 2017 - 3:01pm

Favourite Math Tricks And Shortcuts!

I didn’t have the easiest time in my grade-school math classes. I had to work a little harder than the other students just to grasp the concepts, but when I did, my goodness it felt like a real achievement. I felt so capable, so versatile, so ready to learn more!

Typically, when I’d made a mathematical breakthrough, it had to do with having learned some sort of analogy, system or trick to help me understand. All the way up until university, I could be counted on to use my fingers and whisper to myself while I took a math test.

Here are some great tricks that many math teachers have shared with me over the years:

The Nines Trick

If you need help remembering your 9 multiplication table, try out this little trick. Let’s say you want to multiply 9 x 3.

  1. Hold up all ten fingers and imagine they are numbered 1 through 10.
  2. Bend down the third finger
  3. What you’re left with is 2 fingers on one side of the bent finger and 7 fingers on the other side.
  4. And 27 is what you get when you multiply the number 9 by 3!

Test this trick out with another problem, like 9 x 7. What number do you get if you bend the seventh finger?

Divisibility Rules

You have 432 doughnuts for your party and you would like each person who attends your party to have 3 doughnuts, how many people can you invite to your party?

When it came to math problems like this, I resorted to counting it out on my fingers until I was introduced to these rules of divisibility (there are few versions of the divisibility rules, but these ones have worked the best for me).

Number 2 is your friend, as long as an even number is on the end.

Number 3 will work for me, if the sum of the digits divides by 3.

Number 4 isn’t a chore, if the last 2 digits are divisible by 4.

Number 5 is a great hero at dividing numbers that end in 5 or 0.

Number 6 goes in to me if I can be divided by 2 or 3.

Number 9 works just fine, if the sum of the digits divides by 9.

Number 10, you will know, works best with numbers ending in 0.

So, since 4+3+2=9 and 9 is divisible by 3, you can divide 432 by 3 to get 144. That's a big party! 

The Percentages Trick

We use percentages all the time, so knowing how to calculate percentages is important to understand. Still, I considered them pretty complicated, until I learned a few tricks for simple percentages.

Let’s say you want to find out how much of a discount you’re going to get on all those doughnuts. 432 doughnuts are going to cost $180 and, since you are buying so many, you are given a 20% discount on those doughnuts.

If you consider that 20% is out of 100 and 100 is divisible by ten, if you divide both numbers by 10, you get, 2 and 18.

Take 2 x 18 = 36

So, you are going to receive a discount of $36 on your doughnut purchase!

Say you wanted to but party favours as well and they cost $12, and they are on sale for 30% off! It’s a great deal but how much will you save?

If you divide by 10, you get 1.2 x 3 = 3.6. So, you will save $3.60.

This trick can get even more exciting and useful the more you learn about it. If you want to get better at figuring percentages in your head, check out this video.

Still curious? Read about how to trick your chess opponents out of all of their gold in, Carl Sagan and the Chessboard. What tricks and shortcuts do you use in your everyday life? Add your tips in the comments! 

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