Comments on: Multiplication Made Easy

By: ctreit

ctreit — Wed, 23 Dec 2009 15:01:23 +0000

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.

By: CarA

CarA — Sun, 20 Dec 2009 18:49:00 +0000

Yes. I must say that is helpful. I would normally just grab my calculator. Great tip, thanks.

By: mamafitz

mamafitz — Fri, 18 Dec 2009 15:49:51 +0000

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.
.-= mamafitz´s lastest post ..Racine Meet =-.

By: Simon@RealmOfProsperity

Simon@RealmOfProsperity — Tue, 15 Dec 2009 23:40:41 +0000

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.

By: Stuart

Stuart — Tue, 15 Dec 2009 00:18:42 +0000

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. .-= Stuart´s lastest post ..New Wireless Competition Approved in Canada! =-.

By: Adam

Adam — Mon, 14 Dec 2009 14:43:01 +0000

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!

By: Kevin

Kevin — Mon, 14 Dec 2009 14:37:53 +0000

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).