Multiplication Is Not Repeated Addition

On the NCTM discussion site myNCTM, there’s currently an extended discussion about “Division and multiplication of fractions.” As the discussion has continued, I’ve grown concerned with what I see as a fundamental problem with the way we often introduce multiplication as repeated addition: “Multiplying 4 by 5 means we’re combining five groups of four items. How many do we have altogether?”

This approach sets our students up for problems when they start to consider multiplication and division with fractions. What could it possibly mean to “combine two and two-thirds groups with four-fifths of an item in each group?”

Keith Devlin wrote a series of columns on this for the MAA, the last of which is here: What Exactly Is Multiplication? I am convinced that Devlin and many others who’ve thought deeply about this are correct: teaching kids something that’s wrong and that they’ll have to relearn as soon as they go beyond whole-number arithmetic is a mistake. Even young students understand the ideas of stretching and shrinking, and exploiting these intuitions when we teach multiplication will serve them much better in the long run.

The myNCTM discussion was kicked off with this question: “Is there a way that is internally consistent to model division of fractions for both a whole number divided by a unit fraction and a unit fraction divided by a whole number?” The post challenged us to provide similar explanation for how to compute 4 ÷ 1/3 and 1/3 ÷  4. Fortunately the discussion didn’t devolve into a debate about memorizing “Ours is not to reason why; just invert and multiply”.  (Nix the Tricks does a good job of explaining why this is a bad idea.) Still, the discussion didn’t move in the direction of describing how a scaling approach provides students with a way to relate multiplication and division, to understand the role of multiplicative inverses, and to use one simple model to compute both compute 4 ÷ 1/3 and 1/3 ÷  4.

Showing is better than telling, so here it is: Below (and here) is a Web Sketchpad double number line model for multiplying and dividing whole numbers and fractions:

Task 1: Play with the number lines: Set different scale factors (n = 2, d = 1 is a good start). Show the control points and experiment to find out what they do.

Task 2: Use these number lines to illustrate two multiplication problems: (a) multiply two whole numbers, and (b) multiply a whole number by a fraction.

Task 3: Illustrate two division problems: (a) divide two whole numbers, and (b) divide a whole number by a fraction. Notice that you can drag the red and green arrows onto each other to show a corresponding problem pair (one multiplication and the other division).

Task 4: Illustrate these two problems from the myNCTM discussion: (a) 4 ÷ 1/3 and (b) 1/3 ÷  4.

I’d love to receive your comments about this model. How well does it connect multiplication and division? How can it help students understand multiplicative inverses? How hard is it for students to figure out how to use the denominator points? How can the model be improved, or how would you like to see it extended?

5 thoughts on “Multiplication Is Not Repeated Addition”

  1. I think this model is terrific as a representation. Repeated addition is a problem for so many, but that’s different than the structure of groups of the same quantity. I love the CGI approach that this situation gives us multiplication, and partitive and quotative division. It extends flexibly to fractions.

    The MyNCTM link: https://my.nctm.org/communities/community-home/digestviewer/viewthread?MessageKey=4d1a5476-a6ad-4bb7-bdbb-dbffa9ddfbd6&CommunityKey=0a10e770-ceb6-4514-ac1c-413a77367143&tab=digestviewer#bm4d1a5476-a6ad-4bb7-bdbb-dbffa9ddfbd6

  2. I blogged about the Devlin articles extensively ten years ago. Here are links in chronological order, allowing readers to see the change in my perspective. I subsequently founded an email group with Keith Devlin for people interested in the question of whether multiplication was “just repeated addition.” Very rarely, something pops up there, but I think for the members, no one seriously accepts the idea that we must or should teach young children that multiplication and repeated addition are the same thing mathematically.

    http://rationalmathed.blogspot.com/2008/07/devlin-on-multiplication-or-what-is.html (in which I challenge Keith Devlin’s take)

    http://rationalmathed.blogspot.com/2008/07/more-on-devlin-and-nature-of.html (a quick follow-up piece)

    http://rationalmathed.blogspot.com/2008/09/does-this-mean-that-phone-company-did.html (in which I change my mind!)

    http://rationalmathed.blogspot.com/2010/02/keith-devlin-extended.html (two years later)

    http://rationalmathed.blogspot.com/2010/03/terezinha-nunes-and-peter-bryant-dole.html (from a key source for Devlin’s original post)

    And just for fun, something I wrote more recently on ways to look at multiplication:
    https://mathblog.com/multiplication/

  3. It would be useful if you gave a link to this discussion. I haven’t been following NCTM stuff for a while and was unable to turn up the conversation to which you’re referring.

    Thanks.

    1. Michael, if you’re logged in to NCTM I believe you can find the thread from this page: https://my.nctm.org/browse/allrecentposts. In fact, once already logged in you might be able to get directly to the thread from this url: https://my.nctm.org/communities/community-home/digestviewer/viewthread?GroupId=205&MessageKey=30e6413e-b80c-4795-b661-475f78b3807e. It’s also possible that in addition to being logged in, you might also have to be signed up for the myNCTM discussion for either of these to work. Sigh!

      1. Thanks, Scott, but it tells me I can’t view that conversation and searching doesn’t bring it up. I’m probably blackballed by NCTM on some level or other for basically telling them to collectively piss off perhaps five years or so back. Saved me a good deal of $$ I could ill afford to send them. Since they killed the Math Forum, they’ve been effectively dead to me, but in many ways it’s been mutual since they backed the Common Core Standards with no caveats at all. I’d been on NCTM’s case for a long time regarding their cluelessness in fighting the Math Wars effectively. I should have realized that at the level of the NCTM board, there was no interest in my perspective.

        If you want to share the links I posted to my blog pieces with folks in that conversation, I’d appreciate it. At least some of my mathematical views are probably not entirely radioactive. 🙂

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