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News from Mathnasium of Mill Valley

Multiplying by 11

Feb 3, 2019

It’s very easy to multiply any number by 11 if you know this little trick.  For example let’s take the number 43. Add the numbers 4 and 3 and you’ll get 7. Then put the number 7 between the numbers 4 and 3. And you’ll get the result 473.   
Now let’s extend the challenge and do the same with the number 78. Add the numbers 7 and 8 and you’ll get 15. But then don’t put the number 15 between the numbers 7 and 8, instead put the second digit, number 5, between the numbers 7 and 8 and add the first digit, number 1, to the number 7. You’ll get the result 858 which is the correct result.