Fractious fractions

Imagine that you’ve got two pizzas…mmm…but you’ve got to share them between three people (boo), and share them fairly, how much does each person get?  With this apparently simple question started the ‘big maths cpd ruckus of 2015.  Teacher A said of course you got two thirds of a pizza whereas teacher B said you got a third of the pizza and apparently it all got quite heated and they ended up having to get some paper plates and cut them up in order to prove who was correct. Which of course they both were.  It’s a determiner problem ( Oh no, not grammar and fractions…is this the new curriculum gone mad?)  Teacher A said two thirds of a (i.e one) pizza, whereas teacher B said one third of the pizza ( i.e one third of two pizzas). Which just goes to prove that fractions are tricky. They are tricky because they aren’t really numbers at all ( ok ok maths peeps, I’m playing fast and loose with definitions here, but I’m talking from the perspective of primary kids ok).  Up until we introduce fractions, when we talk about numbers, we mean natural numbers……the counting numbers. If I show a child a 2 I can get 2 pencils or 2 chairs to illustrate what I mean. Whereas if I show a child 1/2…well I could show them half a pizza or half a triangle but 1/2 is so much more loaded than that. Back to determiners. If I show them half of a pizza, I am actually saying – look- here is one pizza. Now let me divide it into two pieces. Each of those two pieces can now be called ‘ a half’. The difficulty being that if I were to have two pizzas to start with, then a half would be one pizza. So  sometimes a half is half a pizza and sometimes it is one pizza or 8 pizzas or…..well pretty much any number could be half of the pizzas.depends on how hungry you were in the first place. Fractions are not naturally numbers at all- quite often they are an instruction to do something: veritable bossy verbs. 1/2 really means ‘divide 1 by 2.’  It also means, ‘ how much you end up with when you divide 1 by 2.’ Which is an adjective apparently ( although it feels like a noun to me).  The answer and the operation are the same, which seems a little tautologous.  And when you cut three things into four equal pieces then the answer  is 3/4 of one thing- even though you started with three things.

The technical term for this is that 1/2 and other fractions are not natural numbers but rational numbers (ratio-nal numbers; they describe ratios). You have to know what the whole is for them to make sense. The whole is not always one. But the final answer is always expressed with reference to one, regardless of the size of the original whole. But we don’t make that clear. Or appreciate that clearly ourselves. Hey, I’m confusing myself writing this!

My better half (little fractions joke there) was recently at some training where they said that children learn about fractions more effectively if taught it in the context of division. Now a reference would be nice here, but husband can’t remember. Did I say he was my better half? It was probably Nunes, it always seems to be Nunes. Anyway – that got me thinking.

A while back resourceaholic posted something by some other secondary teacher (which of course I can’t find) about teaching  addition of fractions with different denominators by using Playdoh . Each pot of Playdoh was divided into different amounts and rolled into little balls of equal size. Because the different denominators are obviously different sizes, the students are less likely to make the error of adding different denominators together. So that made me think that at primary level maybe we should start teaching fractions by just investigating different denominators – and not mention numerators AT ALL until children clearly get that larger denominators means smaller pieces. In terms of writing, we would just write the / line ( that’s probably got a special name…) and the denominator underneath.   I imagined telling the children that the vinculum ( thanks google) aka the fraction bar or slash was a knife that cut the Playdoh ( or whatever) into a certain number of pieces and that we could record how many pieces by writing that number underneath the knife slash. Writing nothing at all above the line.  Then, after many happy hours slashing Playdoh into different amounts and becoming really fluent at writing these (sans numerator) and at naming these,  I finally pose the question- as if it has just occurred to me- ‘ but what if I wanted two of these things called fifths, how would I write that?’ And hopefully someone would suggest we could write that number above the knife/slash/fractions bar/vinculum. And the entire class would henceforth have a rock solid understanding of what a denominator is and how it is logically prior to the numerator- which is a mere adjective to the denominator’s  verb.

Because playdo is sticky, it also makes it easy to divide say  3 tubs of playdo into 4, by firstly amalgamating the 3 tubs worth into one great big ball and the dividing it into four, and then also by dividing each tub into four and then amalgamating one piece from each- and realising this is the same.  And of course it allows us to see that half of one tub is half but half of 4 tubs is 2 and finally that 1/3 of 2 tubs is 2/3 of 1 tub.

Edit: March 2016. My colleague Keeley Warren was much taken with this idea and spent a couple of days with her year 3 class and some play dough dividing it into different denominators….or ‘doughnominators’ and recording these saying the vinculum was the knife and not putting any numerator above it.  As a result, the entire class quickly grasped that the bigger the denominator the smaller each piece is.  Then when she returned a few weeks later to do fractions again- they had no problems whatsoever and grasped everything really quickly. She was amazed at how well it worked and it is definitely going to be standard procedure here from now on.

But that was before I heard about the ‘learn fractions as division’ thing. Which made me want to adapt this lesson further and start with bigger numbers. In fact, to teach division as sharing whole numbers using the vinculum thingy from the off- again saying it’s the knife that divides thing up – or pushes them into piles…..two for you and two for me. Sticking to even numbers initially but them moving onto dividing one thing by two, four etc.  Meanwhile division as grouping could be represented using the other sign ( the obelus, apparently) which keyboards  don’t tend to let you do- certainly not iPads on WordPress- but you know the one I mean. Not quite sure how and when you’d tie the two together yet. Via arrays probably. Love a good array.

Haven’t actually taught it like this- but I think it might work well. One problem though is that the numerator means slightly different things in each case and teachers would need to be very clear in their explanations. In traditional fractions teaching, the denominator indicates the number of equal parts into which a whole was cut and the numerator indicates the number of parts taken.  In division, the numerator refers to the number of items being shared and the denominator refers to number of recipients: if 2 chocolates are shared among 4 children, the number 2 refers to the number of chocolates being shared and the number 4 refers to the number of recipients; the fraction 2/4 indicates both the division – 2 divided by 4 – and the portion that each one receives.  It’s two quarters of one chocolate. Not of two chocolates. Our fraction is now an adjective, or arguably a noun, with its own special space on a number line. Hey it could probably grace the back of a football shirt.

Ps. Here’s that reference. It was Nunes! 

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Fractious fractions

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