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Multiply a 2-digit number by 9 (An Alternative)

Updated on April 19, 2019
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9 is an interesting number. It is the last number in the series of single digits. It is revered in Hinduism as it is considered as a complete, perfected and divine number because it represents the end of the cycle in decimal system.

There are many significant facts about 9 in various fields.

Let us not delve into all those aspects.

Back to main topic:

Take a simple 2-digit number: 12

12x9= 108, this is simple math as you may have known this answer without even calculating it due to day-today usage.

How is it calculated in a traditional way:

12

x 9 .............................Step 1: you proceed from right to left. 9x2=18.

--------

8 ..............................carry over 1

..............................Step 2: 9x1=9 carry over 1 is added to this...so we get 10

Hence the final answer is 108

12x9=108


Let us look at an alternative way of doing this:

1)12x9

Step1: The 2-digit number is 12. So, do this simple subtraction by subtracting the 2 from 12: 12-2=10

Note: the subtracted digit 2 comes from this: Extract the 1st digit i.e., 1 of 12. Add 1 to it.

Step2: Take the last digit of 12:2. As we know 9x2 yields 18, whose last digit is 8. Consider this 8 as an answer of this step.

Step3: Combine the answers of Step1 and Step2. We get: 108


2) 14x9

Step1: The 2-digit number is 14. So, do this simple subtraction by subtracting the 2 from 14: 14-2=12

Note: the subtracted digit 2 comes from this: Extract the 1st digit i.e, 1 of 14. Add 1 to it.

Step2: Take the last digit of 14: 4. As we know 9x4 yields 36, whose last digit is 6. Consider this 6 as an answer of this step.

Step3: Combine the answers of Step1 and Step2. We get: 126.


3) 67x9

Step1: The 2-digit number is 67. So, do this simple subtraction by subtracting the 7 from 67: 67-7=60

Note: the subtracted digit 7 comes from this: Extract the 1st digit i.e., 6 of 67. Add 1 to it.

Step2: Take the last digit of 67: 7. As we know 9x7 yields 63, whose last digit is 3. Consider this 3 as an answer of this step.

Step3: Combine the answers of Step1 and Step2. We get: 603


4)99x9

Step1: The 2-digit number is 99. So, do this simple subtraction by subtracting the 10 from 99: 99-10=89

Note: the subtracted digit 10 comes from this: Extract the 1st digit i.e., 9 of 99. Add 1 to it.

Step2: Take the last digit of 99: 9. As we know 9x9 yields 81, whose last digit is 1. Consider this 1 as an answer of this step.

Step3: Combine the answers of Step1 and Step2. We get: 891


You can take any 2-digit number(except the ones ending with 0) and try this out. This method may seem cumbersome in the beginning. But as you practise it, you will find it so simple. You may actually find it easier to use this method than the traditional way of multiplying.


Comments

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  • poojasd7 profile imageAUTHOR

    poojasd7 

    8 years ago from India

    @Rahul, When I come up with more such stuff, I shall patent them.

    Thanks a lot. :-)

  • rahul0324 profile image

    Jessee R 

    8 years ago from Gurgaon, India

    Pretty interesting... I think you should patent this theorem by your name :)

  • poojasd7 profile imageAUTHOR

    poojasd7 

    8 years ago from India

    @jellygattor, I am so happy to know that you found this trick useful. Thanks a ton :-)

  • jellygator profile image

    jellygator 

    8 years ago from USA

    Very nice! These kinds of tips are great for helping children learn math easier, and that seems to be more important all the time.

  • poojasd7 profile imageAUTHOR

    poojasd7 

    8 years ago from India

    @stephaniedas I am glad to know that this trick will help you. Thanks for visiting this hub :-)

  • stephaniedas profile image

    Stephanie Das 

    8 years ago from Miami, US

    Wow! Never knew this...this will help in my job where I work with teenagers. Most of the tutoring is in math anyway!

  • poojasd7 profile imageAUTHOR

    poojasd7 

    8 years ago from India

    @Nate, yes it is. thanks for stopping by and commenting on it.

  • NateB11 profile image

    Nathan Bernardo 

    8 years ago from California, United States of America

    Good trick, makes multiplication easier; and it would be a good teaching tool!

  • poojasd7 profile imageAUTHOR

    poojasd7 

    8 years ago from India

    @Sofija, :-) yes they are really cool and simple!

  • diplorging profile image

    diplorging 

    8 years ago from Serbia

    I really like this little tricks for faster calculations

  • poojasd7 profile imageAUTHOR

    poojasd7 

    8 years ago from India

    Dear Janine, Thanks a lot :-)

  • Janine Huldie profile image

    Janine Huldie 

    8 years ago from New York, New York

    Poojasd7, no problem and pinning this for future reference and seriously awesome!!!

  • poojasd7 profile imageAUTHOR

    poojasd7 

    8 years ago from India

    Oh wow Janine, I feel honored to have appreciated my math technique by a Math Teacher. :-)

    Thanks

  • Janine Huldie profile image

    Janine Huldie 

    8 years ago from New York, New York

    The math teacher in me loved this Hub. Thanks for sharing this awesome concept with the number 9 and am also sharing and voting up!!

  • poojasd7 profile imageAUTHOR

    poojasd7 

    8 years ago from India

    Thanks Anusujith :-)

  • anusujith profile image

    Anoop Aravind A 

    8 years ago from Nilambur, Kerala, India

    Interesting Hub...

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