Sunday, February 12, 2012

I will try to post interesting math problems and troublesome topics for students. 

Quick mental math. 11 X 22 = 242, 11 X 27 = 297, 11 X 45 = 495..... These are examples of multiplication of a 2 digit number by 11. To get the answer in your head, the first digit in the answer is the first digit in the number being multiplied by 11, the middle digit is the sum of the 2 numbers and the last digit is the last digit in the number. Another example.. 11 X 67.. first digit is 6 the sum of the digits is 13, so carry the one to make the first digit 7 and the middle digit 3, then drop the 7 to get 737.

Feel free to ask me any math question that comes up.


  1. I love that 11's trick. There are many other mental math tricks like that. Too bad most children are not taught them in school. It would make learning math a little more fun for them. Hopefully inspiring them to want to learn more of this "magic" and "poetic" art we call math.

    I have a similar method for multiplying by 9,99,999...etc you can check it out here if you like
    Math Journal multiplying by 9's

    1. John, thanks for the reply! Yes, I like the one with multiplying by 9's too, thanks for posting a link to that one. There are many other mental math tricks that I have come across in the book, "Secrets of Mental Math" by Arthur Benjamin and Michael Shermer