Thursday, May 13, 2010

Multiplying Fractions, Times Three

In a gig with Mr. Schiller's 5th grade last week, I was overcome with indecision.  They'd been working on fraction multiplication and I had three related activities, and asked the teacher to pick based on what he wanted for the class.
The first was just skill practice.  (This is the option Mr. Schiller chose.)   I had never made it, but was confident that you could make a good fraction version of the Product Game.  This is almost what I made:
The only change is that I originally left off 5/12, but the fifth graders convinced me that it should be on.  My thinking was that it would be nice if one unsimplified product was not available, creating a situation where players had to consider equivalent fractions.  But the game itself brought it up enough that I think it's unnecessary.  Click on the image for the full size image which should print properly.

These students have been practicing fraction multiplication, and simplifying and 'unsimplifying' the result.  They have played the Product Game, which is the greatest math practice game ever.  (In the Connected Mathematics Project Prime Time module now, may be from the Middle Grades Mathematics Project before that.)

I launched the game by playing me vs. the class.  Reemphasizing that you only get to change one factor at a time, the goal is to get four in a row, and the new idea that there are equivalent fractions.  If you multiply and get 6/12, you can cover 1/2, and vice versa.  Then the students played pair vs. pair.  At the end we summarized by discussing what they noticed about the game, and what they thought made for a good strategy.  I did point out to them that someone would tell them fraction division was hard, but they've already done it when they're figuring what to multiply 3/4 by to get 6/12.

The second idea was to have the students develop the ability to make sense of their answers through a constructive representation.  Right now, I think the students are mechanically carrying out the multiplication, without much intuition to inform them if their answers are sensible.  These questions are adapted from an activity I do with my preservice elementary teachers.

I like playing the video before the activity, but that is obviously optional.

"Potatoes, mash em, boil em, stick em in a stew.” – Samwise Gamgee.

Things are _______ (awful, bad, okay, good, great) because you have potatoes!  Draw a picture to justify each answer.  Write an equation or number sentence for each story, if you can.

Find how many pounds of potatoes you’ve got if you search the cupboards and find…
1)  1/2 a bag of potatoes, which started with 2 pounds of potatoes.

2)  1/2 a bag of potatoes, which started with 2/3 pound of potatoes. 

3)  3/4 a bag of potatoes, which started with 2/3 pound of potatoes. 

4)  1 ½ bags of potatoes, which each started with 2/3 pound of potatoes.

5)  1 ½ bags of potatoes, which each started with 3/4 pound of potatoes.

6)  4 bags of potatoes, which each started with 1 1/3 pound of potatoes.

7)  2 2/3  bags of potatoes, which each started with 3 ¼ pound of potatoes.

8)  ____ bags of potatoes, which each started with ____ pounds of potatoes.
(You make the problem!)

The numbers are chosen pretty intentionally to allow for some connections and the possibility of relating the quantities to each other.  I like potatoes (of course) because they can be used for a discrete or an area model or a nice casserole.  My plan was to start with problem 2, demonstrating for the class a couple different models, and then have them start on number 1.

The third option was to get at a new context for multiplication.  As anyone following the Keith Devlin multiplication fiasco knows, the prevalent contexts for multiplication involve repeated groups.  One of the other contexts that is often nice for rational numbers is the idea of stretching and shrinking.  That always puts me in mind of Alice, and how her terrific adventures began.

 Go Ask Alice
“One pill makes you larger, And one pill makes you small
And the ones that mother gives you, Don't do anything at all
Go ask Alice, When she's ten feet tall” – Jefferson Airplane

Sort of from “Using Alice in Wonderland to teach Multiplication of Fractions,” Susan Taber, MTMS, Dec 2006

“There seemed to be no use in waiting by the little door, so she went back to the table, half hoping she might find another key on it, or at time she found a little bottle on it, ('which certainly was not here before,' said Alice,) and round the neck of the bottle was a paper label, with the words 'DRINK ME' beautifully printed on it in large letters.”  Alice in Wonderland, Lewis Carroll.

It turns out that she drinks it, and shrinks to 1/6th her former size.  Now she later finds a cake…
“She ate a little bit, and said anxiously to herself, 'Which way? Which way?', holding her hand on the top of her head to feel which way it was growing, and she was quite surprised to find that she remained the same size: to be sure, this generally happens when one eats cake, but Alice had got so much into the way of expecting nothing but out-of-the-way things to happen, that it seemed quite dull and stupid for life to go on in the common way.”  But soon, “Just then her head struck against the roof of the hall: in fact she was now more than nine feet high, and she at once took up the little golden key and hurried off to the garden door.”

It made her grow almost 12 times larger.

1)    What height would she be if at 10 ft tall she took a sip of on-sixth potion?

2)    If she started at 5 ft tall, and then took a sip of one-sixth potion, how tall would she be?  In feet?  In inches?

3)    If she took a bite of ten times cake and then a sip of potion, would she be the same height as what she did, which was take a sip and then take a bite?

4)    Having had one sip and then one bite, how close can she get back to her original size?

Let’s add to the story shall we?  Suppose she finds a times-three cookie, and a one-fourth soda.
5)    Starting at 5 feet, what height does the one-fourth soda make her?

6)    What effect would the times three cookie have, followed by the one sixth potion?

7)    If she starts at 5 feet and wants to be 6 feet tall at the end of it, what should she eat?

8)    In the story she wishes to pass through a 15” door.  If she had the choice of all four magic items, what should she do, starting off at 5 feet tall?

9)    What other mixtures are possible with all four items?

What does this have to do with fraction multiplying? 

What did you learn from these problems?

It's just such an amazing context, and that's without getting into the mushroom, which she uses for more controlled growing and shrinking later on.  Of course when my son is ready for these problems (soon, I think) we'll have to switch the context.

He shrinks, too.  Perfect!


  1. and that's without getting into the mushroom, which she uses for more controlled growing and shrinking later on.

    For the right students, that would be a great way to scaffold logarithms and related subjects like compound interest, but that's a long way from just multiplying fractions.

  2. Ben, that's the birth of a great problem. If every nibble grows/shrinks Alice 25%... the scene where she's stuck in the hare's house is the passage to start.