Instructables
Picture of How To Multiply Any Number by 11 Mentally
This is a guide on how to easily multiply any number by 11 mentally and without the aid of a calculator. It can be a cool party trick or maybe you just need to multiply by 11 in a pinch. You never know, maybe after learning this trick the ladies(gents, whatever you're in to) will come flocking....or not.
 
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Step 1: Limitations and Precautions

Picture of Limitations and Precautions
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There is no real limit to what number you can multiply by 11 with this trick. Just be aware that as you move into larger numbers, they become a little more time intensive. The only background I would suggest to have in this trick is be really familiar with your basic addition tables and be quick. The quicker you are with simple addition, the quicker this trick goes.

Step 2: The Trick

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Most people I know will multiply 11s but doing the 10s method, which is pretty fast. You add some zeroes and add itself back in; nothing exciting. For smaller numbers this is a decent method but what if you wanted to do larger numbers?- thats where this comes into play.

The Trick:

1. Pick any number to multyply by 11.
ex. Im going to pick 3,516.

2. Take your number that you have picked and write down the last number(the ones digit).
ex. in our example its 6.

3. Then starting from the last two digits(ones and tens) add them together and write that number next to your last number.
ex. 1+6=7. Then 7 is placed next to my 6 therefore 76.

4. You would continue this process from right to left until you have no more digits to add.
ex. 5+1=6. Then 6 is placed next to 76 therefore 676.
ex. 3+5=8. Then 8 is placed next to 676 therefore 8676

5. The final step in the process is to place the foreleading digit in front of the number youve created through all of the addition.
ex. 3 is placed next to 8676 which gives you 38,676.

So there you have it. You can check that on the calculator if you would like and this will work for all numbers multiplied by 11. Good luck and happy arithmetic!
admin6 years ago
Hey, this is a great instructable and is very informative. Just one thing is missing... pictures! It really helps a lot when trying to follow directions so you should consider taking some photographs. Once you do that and leave me a message when you have so that we can publish your work. Thanks! Thanks for the cool instructable and we hope to publish this soon!
tristinGrind (author)  admin6 years ago
Pictures added!
Spypro5 years ago
very nice :)
macmaniac5 years ago
It's not doing it mentally if you have to write stuff down
tristinGrind (author)  macmaniac5 years ago
I only tell you to write it down in the beginning to show you how it works. I dont write anything down as Ive been doing if for quite some time now. Its the same as when youre a kid... you write everything out, but as you get better you can do it all in your head; same concept.
R0B0_Ninja5 years ago
Very well explained. Simple, yet fascinating stuff like this could make more people be interested in math.
cool thanks