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PostPosted: Fri Nov 16, 2012 12:24 pm 
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Hi, I am struggling to teach DD ratios and think I am making it over complicated..

can any one please recommend the best way:

so if the question was:

6 oranges cost 90
how much will 10 oranges cost:

what steps should I use,

I know I would do 10/6 * 90 but how do I explain WHY to do this to DD...

Thanks in advance
SleepyHead


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PostPosted: Fri Nov 16, 2012 12:34 pm 
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Location: Shamballa
Hi

If 6 oranges cost 90p

Then 1 orange will cost how much ?

Have to divide both sides by 6.

so 1 orange will cost 90/6p

10 oranges will cost 10 times as much 10* 90/6p

100 will cost 100* 90/6p

not sure if this helps ?

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"To err is human;to forgive ,divine"


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PostPosted: Fri Nov 16, 2012 12:36 pm 
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great thanks so always find 1 first...


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PostPosted: Fri Nov 16, 2012 12:37 pm 
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Location: Warwickshire
To keep it simple I'd do this as:
cost of 1 orange = 90/6 = 15
so 10 oranges = 10 * 15


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PostPosted: Fri Nov 16, 2012 12:40 pm 
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Yes. Thats the way I teach my dd.

also you have questions such as:

Car travels 15km in 20 mins . How far will it travel in 30 mins.

in 20 mins travels 15km
1 min travles 15/20 km

30 min travels : 30 *15/20

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PostPosted: Fri Nov 16, 2012 12:48 pm 
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Question:

Car travels 15km in 20 mins .
How far will it travel in 30 mins.

Distance Time
15km 20min
? 30mins

so would say 30/20 * 15

Is that the same thing

if we had to find out distance in 8 mins

then 8/20 * 15


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PostPosted: Fri Nov 16, 2012 12:56 pm 
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Location: Shamballa
it's the same thing.

my gut feel is that children understand the approach below more intuitively:

work out how much 1 unit costs
or work out how far the car travels in 1 minute

then multiply by the number of units or time travelled ( in minutes)

with harder problems you need to ensure that you have the correct units:Minutes,hours,KM,m etc

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PostPosted: Fri Nov 16, 2012 1:03 pm 
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ok so applying it to a question from S and S (bk 3)

Tape costs 12p for 50cm.
How much does 125cm cost?

would you still work out for 1cm? because you could say that 125 (= 50+50+25) therefore (12p + 12p + 6p)

ie 1cm costs 12/50

then 125cm costs 12/50 * 125

thanks..
SH


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PostPosted: Fri Nov 16, 2012 1:23 pm 
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In that case I'd probably do it per 25cm.


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PostPosted: Fri Nov 16, 2012 1:25 pm 
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thank you.


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