How can I explain this maths question to my year 3?

[fatbananas]

Hi,
My year 3 DS is slowly making his way through the Schofield and Sims mental arithmetic book 2. It's proving really helpful in getting to grips with word problems. However, he got stuck on the question below and, although I could work out the answer, I couldn't explain how I got there . Can anyone help me explain it to a year 3?

Find the cost of 100 grams if 1/2 kg costs 40p.

Thank you!

[guest201]

I would start asking how many grams are in 1/2 kg which is 500

if 500 grams cost 40p
divide by 10
50 grams cost 4p
double it 100 grams cost 8p

[Belinda]

.

[fatbananas]

Thank you. Yes, I thought it was ratio as well. Do they do ratio in year 3? I was told that the S&S mental arithmetic book 2 was appropriate to year 3 and he's largely been fine but it was one of two questions at the end of 10 questions that he had trouble with and, presumably, they get progressively harder as you go through the test/ book.

Anyway, thanks for that. Think I might go back to basics with him with ratio and then look at that question again.

Thank you Guest201 and Belinda!

[Belinda]

.

[mystery]

Yes I like all those methods. Choosing to work out the cost of 100g first of all might puzzle him - but it might not. How about getting him to think about the cost of 1g first of all?

So, 500 g costs 40p
1g costs 40 / 500 pence

400g costs 40 / 500 x 400 (sorry can't lay it out nicely using this keyboard)

If he can't work that out in head or with pencil and paper - it's same as 40 / 5 x 4 = 8 x 4 = 32, then give him a calculator - even if he can't handle the numbers he can understand what he needs to divide and multiply by to get the right answer.

You could invent loads of different questions of the same type, but with smaller numbers, to see if he understands what he needs to do with what.

e.g. 7 gobstoppers cost 35pence, how much would 5 gobstoppers cost?

Then move on to ones like the initial question where it is easier to write down how you would work out the cost or weight of one thing, but not actually work it through but save all the multiplying and dividing to do later on in one fell-swoop.

[fatbananas]

Thanks Mystery. Having just read the "Tiffin School - Admissions Arrangements" thread about the lack of secondary school places around here, think I'll be home-schooling at secondary level, if I don't get the maths up to gold standard!