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February 11, 2010

Mulitpling by 5s

I work with the kids. I give them math problems, I go over the short explanation on how to do it (if needed), and send them on their way. If they NEED help I will help them. However for the most part I leave them to do it on their own, especially Superstar!

Superstar has been working on multiplication this is the typical course work for "3rd graders". We have just finished his multiplication chapters and the next chapters covers things like Geometry (solid and plane figures, triangles, lines, rays, angles, and congruent figures) and Graphing (making and using bar and line graphs).  Once the the Geometry and Graphing Chapters are done we will start working with Division, followed by Fractions and Decimals.

Seeing we are taking a break from the multiplication and it's going to be awhile before we start the dividing I thought I would take advantage of this time and help Superstar memorize his multiplication facts. We will do the book work followed by going over the multiplication flash cards.

We used the flash cards yesterday and he actually knows them pretty well. Some of them he takes longer then I would like to see, but I know with practice he'll know them better and will not need so much time to come to the answer.

We discuss which ones he knows really well and which one he still struggles on. He then shared with me that 5s are easy because of "his method". I asked him to explain his method.
Well it's simple if the number is even you just 1/2 it and times it by 10
  • 16 x 5 = 8 x 10
  • 24 x 5 = 12 x 10
  • 112 x 5 = 56 x 10
  • 6 x 5 = 3 x 10
If the number is odd then you take away 1, 1/2 the rest, times it by 10, then add 5 for the one you took away
  • 17 x 5 = (8 x 10) +5
  • 25 x 5 = (12 x 10) +5
  • 221 x 5 = (110 x 10) +5
  • 3 x 5 = (2 x 10) +5
This method works for ANY (whole) number times 5. We tried it on several numbers. I gave him all kinds of numbers odd and even and asked him to use his method to test it. He was able to give me the answer to any number multiplied by 5 using this method.

I was simply FLOORED that he came up with this on HIS OWN!!! I did NOT teach him this and it is NOT in his book. I didn't know what to think, other then he must just have the brain for math and no wonder it comes easy to him and I don't have to explain much to him. Web-princess on the other hand is a different story. Most of the time I just give Superstar his book and he goes to town. I often do not need to instruct him on what to do.

That's one reason I'm happy I'm homeschooling him because I know he doesn't have to sit through long boring instructions on how to do math problems. He can just get to work on them without me holding him back with long boring instructions. If he doesn't know something or gets stuck I'm here and I DO help him. However if he can look at the book and know what to do then that's fine with me. (Besides it allows me to sit with Web-princess more, without feeling guilty, because she need more help.)

I'm sure that Superstar really didn't come up with this method for the first time, I'm sure there are math wizards out there that know what this method is called.

I'm just amazed that my 8 year old discovered it on his own, without it being taught by me or the book. It's interesting to see how his brain is processing math.


  1. That is TOTALLY awesome!!!!!!!!
    And it proves the point that mathematics is about the CONCEPT, NOT the answer...clearly SS UNDERSTANDS the concept to the point that he can create METHOD!!!

  2. Wow!! Thanks!! This is going to be VERY helpful when teaching my oldest multiplication! I only knew about the "Trick of Nine" (one less than the number you multiply by is the first digit of the answer [for 9x3, 2. for 9x4, 3. etc], the second digit of the answer is how many units that number is away from 9. [9x3=2"7", 9x4= 3"6"). (sorry if that was confusing!)

  3. It wasn't confusing I totally understand what you are saying Mary! Thanks for stopping by today!


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