11 Plus Exams Forum

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I am feeling really thick as was never taught to compare fractions by cross- multiplying.
This is brilliant and am going to teach this to DD's as soon as they wake up - it would have been so useful for all those ordering fractions questions on the 11+ !
Was anyone else taught this?

Sorry - what is good about their suggested method? Please don't teach this!

What makes them an authority on teaching fractions?

The way to compare fractions IS to make common denominators - that is the basis of understanding what a fraction is.

You cannot use their method if you have more than two fractions to compare ....

I too thought the cross multiplication thing was very neat.

However, skip reading the document he was so much against, I thought it was quite good. I remember drawing lots of very similar things out for my daughter when teaching fractions. In my book, the more diagrams the better. A picture is so much more interesting for a child than writing and does help build their understanding if they can "see" why they are doing things. Lots of bright colours helps.

I find, as her understanding has built, the diagrams are not needed and so she would now simply be writing the sum out to find equivalents.

I still use diagrams on other things. For instance, in completing the square, I would draw out a square of length x+b, so she could see the b^2 bit she was taking away.

Famously even Richard Feynman was not adverse to drawing diagrams.

Anyone that has studied teaching and learning maths knows the problems cross multiplying can lead to - that's why it's never taught as a technique.

Understanding is the key to learning maths - fractions are a difficult concept and 'short-cuts' lead to error. There's been lots of research on teaching fractions ...

It will be much simpler for children to learn fundamentals of fractions using LCM of denominators approach that can be applied to add, subtract or compare 2 or more fractions, rather than a technique which can only be applied to compare only 2 fractions at a time.

You can teach them techniques or short cuts, once they have strong understanding of core concepts.

From the article quoted by OP:

One of them said “You cannot teach kids to cross-multiply because then they will do it everywhere, like when they are trying to add fractions”. Occupy Math thinks this is very close to “You cannot teach people to use screwdrivers because they will try to use them to tighten bolts or stir coffee.”

Believe it or not, I did witness a window-fitter stirring his coffee with a screwdriver...

booellesmum wrote:https://occupymath.wordpress.com/2016/0 ... fractions/

I am feeling really thick as was never taught to compare fractions by cross- multiplying.
This is brilliant and am going to teach this to DD's as soon as they wake up - it would have been so useful for all those ordering fractions questions on the 11+ !
Was anyone else taught this?

I completely agree that this is a brilliant method!

olucares wrote:I completely agree that this is a brilliant method!

It doesn't work for more than two fractions - please read the other posts about why it is not taught.

Don't see what's wrong with simply identifying common denimators and the comparing numerators.

The OP was suggesting cross-multiplying which is not a good method.