# Talk

## Y5 - adding fractions with different denominators

(20 Posts)
RubyRedRobin Mon 23-Jan-17 19:18:58

Currently being taught in dc class.

Ds slightly confused in the lesson today so thought I'd go over it with him tonight.

Any Y5 teachers here? I've looked online and there are few different methods. I will speak to the teacher at some point but thought I'd ask here:-

E.g. 2/3 + 4/5

Multiply 3x5 to get 15
Then multiply numerator by opposite denominator

10/15 + 12/15 = 22/15

Ds can convert easily the improper fraction.

Is this the method you would have used?

civilfawlty Mon 23-Jan-17 19:22:15

I'm not a teacher, but it's how I taught my dd. I found she was too young for LCD at that point. Others may know better though.

RubyRedRobin Mon 23-Jan-17 19:27:24

I looked in the Khan Academy site but the method I've used above seemed far more straightforward.

Mummymathsteacher Mon 23-Jan-17 19:31:27

If I were teaching it, I would put the emphasis on having a common denominator. What is the lowest common multiple? Then focus on readjusting the numerators. I always emphasise that it is proportional - if you do something to the bottom, you have to do the same to the top.

When teaching, I avoid saying you times it by the other denominator (even though that works) because I want the children to understand what they are doing, rather th a just following a rule. That being said, there's no harm in describing it like you did, if that helps them understand.

RubyRedRobin Mon 23-Jan-17 19:35:01

Mummy, thank you. That's what I needed to know with regards to ds understanding. That's how the Khan Academy do it, finding the lowest common denominator. I'll use that method.

RubyRedRobin Mon 23-Jan-17 19:36:03

I'm not great at maths and have to fully understand it myself before trying to explain.

Mummymathsteacher Mon 23-Jan-17 19:44:11

I quite like the khan academy videos actually. I sometimes share them with my students. Not sure if it's free, but LearnZillion often make good tutorials too.

Mummymathsteacher Mon 23-Jan-17 19:45:58

Oh and use diagrams of possible e.g. two rectangles, one cut into thirds vertically and the other into horizobtal fifths. When you put them together, it makes it clear how 2/3 is actually 10/15

Ollycat Mon 23-Jan-17 19:48:42

You need to talk about common denominators so that children can understand how fractions work - 1/2 = 2/4; 1/10 = 2/20 - if children understand how fractions come to be they have the skills to work these things out. I work in secondary and you'd be surprised how many students have never learnt the basic principles of things like this.

RubyRedRobin Mon 23-Jan-17 20:00:52

Even my 'Maths for Mum and Dads' book shows the first method I used. I found the first method on YouTube, I sometimes find it easier to watch an example than read it.

I don't want to confuse ds further by showing him an entirely different method but if I cover all bases it should click with him.

FizzySmile Mon 23-Jan-17 20:01:00

This is what I am teaching my Y5/6 class at the moment.
Like Mummymathsteacher I'd teach it as finding the common denominator, or 'bringing it to the the same name'.

Looking at the initial denominators what the lowest number they have in common (considering the times tables)? In this case 15.

So, 2/3 + 4/5
=> 10/15 + 12/15
=> 22/15 => 1 7/15
(I ask my pupils to show each step of working as above so I can spot where they are having difficulty.)

I don't teach it using the multiply across method, as I don't feel it shows the pupils equivalency between the fractions, and therefore adding when denominators are the same.
Sorry if this doesn't make sense, long day!

RubyRedRobin Mon 23-Jan-17 20:04:18

I'll teach him your methods Mummy and thanks Olly, makes sense.

RubyRedRobin Mon 23-Jan-17 20:07:07

Thanks Fizzy. Yes that makes sense.
Off to grab the whiteboard and go over this again.

irvineoneohone Mon 23-Jan-17 20:15:16

try this video.

mathantics.com/section/lesson-video/common-denominator-ecd

And also watch whole fraction section as well for more understanding.

mathantics.com/

OutwiththeOutCrowd Mon 23-Jan-17 21:14:34

It’s interesting that the two methods are difficult to distinguish in the particular case given because the denominators are both prime so the lowest common multiple is the product of the denominators.

An example like 2/3 + 5/6 shows the difference between the two methods.

The product of denominators common denominator method (OP method) gives:

12/18 + 15/18 = 27/18 = 1 ½

The lowest common denominator (from the lowest common multiple) method gives:

4/6 + 5/6 = 9/6 = 1 ½

RubyRedRobin Mon 23-Jan-17 21:41:35

Out, yes I see what you mean. I have gone over this again and ds is great at working out the LCD as he's really secure with his times tables.
He seems more secure now. We'll practise a little each night.

RubyRedRobin Mon 23-Jan-17 21:45:05

Oh and thank you also, Irvine.

katiehigham Mon 13-Feb-17 21:32:23

Message deleted by MNHQ. Here's a link to our Talk Guidelines.

BarFlyInThePeanuts Mon 13-Feb-17 21:41:54

We use the butterfly method. Much easier!

BarFlyInThePeanuts Mon 13-Feb-17 21:43:08

Like this.

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