Sometimes when I am out with friends and family, a medium sized multiplication problem comes about. For some reason, I can do these types of problems in my head fast. I’m not sure where I learned this trick, but I am going to share it with you.
Third Grade Math
I remember back in third grade when we used to play multiplication games. We would compete with other students in the class to see who could learn the basic multiplication tables first. We started with the 2′s (2 x 2 all the way to 2 x 9) and then moved onto the 3′s, 4′s, all the way to the 9′s. The tip that I am going to show you assumes that you already know this basic math.
Multiples of Ten
The process that I use for multiplication involves breaking the problem down into two parts. The first part is a multiple of 10 and the other is the basic math explained above. Let’s look at an example:
17 x 6 =
When faced with a problem like this, I first break down the larger number in my head as a multiple of 10. So, I take 10 out of the 17 and it leaves me with 10 x 6. You can do that in your head and get 60. I then take the remainder of 7 and multiply it by 6. That is a problem from the basic section and it gives me 42. All you need to do then is add the 60 and the 42 and it gives 102. Isn’t that a lot easier to do in your head? Here’s another example just to be sure you got it:
16 x 8 =
First, break the larger number down into a multiple of 10. That will give you 10 x 8 which equals 80. Then take the remainder of 6 and multiply it by the 8. That gives you 48. Add the 80 and the 48 and you get your answer of 128.
What do you think? Easy? Will it be useful to you?
photo by: Sagolla
I’ve known this trick forever too. And I bunch of others too. One of my favorites is for calculating an 18% tip on a bill. 18% is just 20%-2% (and 20% is twice 10%).
So on a $34.20 bill… you take 10% ($3.42), double it ($6.84) and minus 10% of that amount ($6.84-$0.68 = $6.12). With practice, you can manage this fairly quickly. No more reaching for your cell phone’s calculator unless you’re feeling really lazy.
There’s a great book out there (Arithmetrics) that I love. As a teacher, I use some of those tricks with my students all the time. I am not sure it covers the trick you mention above (it probably does but most of the ones I use with my students are for addition and subtraction).
You stole my thunder! I was planning on writing about that trick for tipping. I guess I still could.
I typically tip 15% so it makes that math a little easier. I just take 10% of the bill and then add half of that to the 10%. So, with a $80 bill, I would take 10% and get $8 and then half of the $8 is $4. Add those two together to get your $12 tip. Pretty simple!
Good tips. I have been doing similar things for since I can remember as well. Math is not everyones cup of tea though. Anyone writing or reading about finance is probably already in the know though.
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Many of these shortcuts are taught by admissions test review programs (Kaplan / Princeton Review) for test such as SAT, GMAT, MCAT, and etc. They are an easy way to save time since those tests are often so difficult.
Just that not many people apply them to real world situations.
that’s called partial products, my 4th grader is learning how to do that this year. very handy to know — well, mental math in general is very good to know.
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Yes. I must say that is helpful. I would normally just grab my calculator. Great tip, thanks.
There are lots of tricks I use. For example, when I multiply 16 x 17, I do 20 x 17 and then I deduct 20% (since 16 is 20% less than 20). When I multiply other more complicated numbers, I try to get an approximation by employing similar tactics.