Hi peasandcucumbers:
I'm just a Mum but you raise an interesting question.
I think the first question I have is what year is your DC?
I agree with redskyatnight - that if in class they've been working with fractions.
So they've been introduced to the concept of fractions (so slicing a pizza into 6 slices and then describing one slice out of the six as 1/6th).
If they've also been introduced to the idea of factors and converting fractions with different denominators or reducing down to the lowest common denominator:
so understanding that 1/2 + 1/4 is the same as 2/4 + 1/4 = 3/4
or understanding that 5/25ths = 1/5
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So if that has already been introduced then both questions you gave as examples (especially if the example to the first problem was shown) should be fairly straightforward (and I'm presuming NC L3 equivalent).
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DD1s class has only had brief forays into the world of fractions and most of the kids in her class great 'Now let's work with fractions' with a groan. Because many struggle with their multiplication tables - getting the idea that 5/25ths can reduce to 1/5th is frankly beyond them.
DD1 has been asked to help with fractions work in class recently - and she spent the whole time saying with fractions you have to do the same to the top as the bottom. So if you can see that a number can go into both the top and the bottom, you can divide by that number:
e.g. reduce 9/ 51st to the lowest possible fraction
factors of 9 (3 * 3)
factors of 51 (3 * 17)
numerator & denominator both have 3 in common - divide both by 3 (3/3 is effectively = 1)
9/3 = 3
51/ 3 = 17
so the fraction reduces down to 3/17th
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The problem here is interesting for DD1's class - because some enjoy the maths problem and application of multiplication table skills and others just simply don't understand why on earth you'd need to do this.
In part - they don't see fractions as another form of division or as proportions
In part - the school rarely uses real-life examples (what proportion of the players scored a goal so far this season for example).
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So I think your question totally depends on what background with fractions your DC's class has had. How secure they are with their multiplication facts. How well they know their inverse multiplication facts.
For DD1s year 6 class - it's quite clear that a number of kids would struggle with the problems you gave as an example - but as you'll see from my other posts on maths our school is particularly weak in this area.
HTH