## Only Fractions

My love and I saw Only Yesterday recently. It’s a 1991 Studio Ghibli film, directed by Isao Takahata. It hasn’t had a United States release before, which is a pity; it’s quite good. The movie is about a woman, Taeko, reflecting on her childhood as she considers changing her life. One of the many wonderfully-realized scenes is about ten-year-old Taeko’s struggles with arithmetic. You probably guessed that, as otherwise the movie would seem outside the remit of this blog.

In the scene Taeko has had a disastrous arithmetic test. Her older sister is trying to coach her through how to divide fractions. It goes lousy. Her older sister insists it’s just a matter of inverting and multiplying. This is a useful tip if you understand how to divide fractions and need to keep straight what you’re doing. If you don’t understand, then it’s whatever the modern equivalent is for instructions on how to set a VCR.

Taeko tries to understand one problem. . She pictures it as an apple and draws a circle, blacking out a third of it. She cuts the rest into four equally-sized pieces and concludes that you could fit six slices into the original apple. Her sister stammers over this and fumes. She declares “that’s multiplication!”. She complains her sister isn’t doing the right thing, she’s not inverting and multiplying. I recognize her sister’s panic. It’s the bluster of someone trying to explain something not actually understood, on watching someone going far off the script.

The scene’s filled with irony. Taeko has a better understanding of what she’s doing than her sister has, but never knows it. Her sister understands a procedure but not what fractions dividing signifies. She can’t say why one wants to invert anything or multiply something. Taeko knows what the question she’s asked means, but not how to relate that to what she’s asked to do.

I don’t want to undervalue learning procedures. They’re worth knowing. They are, once you master them, efficient ways to compute. But there are many ways to master a procedure. I can’t believe there is one way to learn *anything* that works for *everyone*. One of many challenges teachers face is exploring the different ways their students best learn. Another is getting close enough to how they best learn that most of the students can understand something. It’s a pity when real people akin to Taeko can’t get that little bridge to connect their drawings of an apple to the page of fractions to be worked out.

## Roger Purves 9:15 pm

onSunday, 27 March, 2016 Permalink |to Joseph:

In the film (*), the problem assigned is to divide 2/3 by 1/4. Takeo is using her drawing of an apple to visualize 2/3 divided by 4. That’s 8/12 divided by 4, or 2/12 = 1/6, Takeo’s answer.

I rarely “invert and multiply”. Instead, I change to fractions with common denominators; in this case 2/3 to 8/12 an 1/4 to 3/12. How many times does 3/12 go into 8/12? More than twice but less than three times; 2 and 2/3 times.

I really enjoyed your post, especially the juxtaposition of “invert and multiply” and “whatever the modern equivalent is for instructions on how to set a VCR”. That made me laugh–gallows humor, though.

Roger Purves

(*) The site blog.wowzers.com (September 11, 2013) has the relevant clip.

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## Joseph Nebus 5:13 am

onWednesday, 30 March, 2016 Permalink |I think moving things to common denominators should help. At least it makes it possible to take the denominators out, and what’s left looks much more like regular old division. But that does also add extra steps to the process, and there is this drive to not do unnecessary steps. But I am biased toward taking extra steps when learning how to do problems.

Thanks so for the kind words.

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## How Much March 2016 Liked My Mathematics Blog | nebusresearch 3:01 pm

onSaturday, 2 April, 2016 Permalink |[…] Only Fractions, talking about a moment in the wonderful movie Only Yesterday that my love and I recently saw. […]

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