The DoughRoller Weekly RoundUp (The 11 Rule in Mathematics Edition)

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Sometimes in life, learning the tricks and the shortcuts is better than knowing the details.  The brain can only handle so much knowledge (although different people have different limits) so if you’ve ever wondered how to multiply a two digit number by 11, wonder no more.  Multiplying two digits numbers in your noggin can take some effort.  Granted, the times when you need to multiply a number by 11 may be few and far between, nevertheless this is how it’s done.

Two simple rules need to be followed and here they are:

  • Rule #1 – If the digits of the number you are multiplying add up to 9 or less, separate the two digits,  add them and place that number in the middle.  You have your answer.

So for example, if we multiply 11 x 81, the first step is to add the 8 and 1 together.  This adds to 9, so we follow rule number one.  Take the 8 and 1, split them up and put the 9 in the middle.  Therefore 11 x 81 is 891.  Written out it can be a tad confusing but once you get a hold of it, you should be able to do this math in your head in a split second.

  • Rule #2 – If the digits of the number you are multiplying add up to more than 9, separate the two digits, add them, place the ones digit in the middle and carry the one over to the first digit.

Again for example, let’s multiply 11 x 76.  Here, the sum of the 7 and 6 is 13, so we can’t just place that in the middle.  What we can do, is put the 3 in the middle, then carry the one over to the 7 and make it an 8.  So our answer becomes 836.  Again, a little complex but once the brain understands it, piece of cake.

11 x 45 = 495 (4+5 = 9)
11 x 32 = 352 (3+2 = 5)
11 x 83 = 913 (8+3 = 11, used the second 1 to place in the middle, carried the first one to the 8 and made 9)

And if you’re multiplying a one digit number by 11, just replace the ones from the 11 with the digit in question.  So 11 x 4 is 44, 11 x 9 is 99 and so on.  Hopefully, you’ll never go through life again wondering how to multiply by 11.

Now for the best money articles from the past week!

Compound Interest Can Be Your Friend @ Cash Money Life: Another cool rule of mathematics, many forget that interest is earned on top of interest, so doubling your money at 2% will take less than 50 years to complete.  (That’s the rule of 72!)

What is Inflation and Can You Avoid It? @ PT Money: They say that only two things are certain in life and that’s death and taxes.  After reading this, you might want to add another one to the list.

Wanna Know the Cheapest Way to Travel the World? @ Budgets Are Sexy: Working for an airline (and making sure you actually on the plane when it takes off) is a great way to see the world one day at a time.  :)

11 Ways the Government is Padding Your Wallet This Summer @ My Dollar Plan: You’ll find a solid list of tax breaks here and you’ll also find a new site design for My Dollar Plan.  Congrats!

Should You Get a Job or Go to Graduate School @ One Money Design: My vote depends on the job you’re after.  If you can obtain a good job before grad school what’s the point in attending.  Don’t get me wrong, higher education is great but you go to school to get the good job.  If you already have it …

5 Reasons To Start Holiday Shopping @ The Street: HA.  I’ve already finished my holiday shopping.  The only problem I have is keeping things a secret for the next 5 months.  I’m not good with secrets.

The Best Credit Cards for College Students @ US News: We guest post on US News regularly and decided to talk about the hot button issue that is college student credit cards.  While it may not be a popular stance, allowing students to manage their credit early and often is better than sending them into the world as credit card rookies.  The earlier the mistake, the more time you have to learn from it.

And this past week we participated in the Carnival of Personal Finance.

Published or Updated: December 6, 2011

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