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how to teach ratios... advice needed!
Posted: Fri Nov 16, 2012 1:24 pm
by SleepyHead
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
Re: how to teach ratios... advice needed!
Posted: Fri Nov 16, 2012 1:34 pm
by IronMikeTyson
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 ?
Re: how to teach ratios... advice needed!
Posted: Fri Nov 16, 2012 1:36 pm
by SleepyHead
great thanks so always find 1 first...
Re: how to teach ratios... advice needed!
Posted: Fri Nov 16, 2012 1:37 pm
by Okanagan
To keep it simple I'd do this as:
cost of 1 orange = 90/6 = 15
so 10 oranges = 10 * 15
Re: how to teach ratios... advice needed!
Posted: Fri Nov 16, 2012 1:40 pm
by IronMikeTyson
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
Re: how to teach ratios... advice needed!
Posted: Fri Nov 16, 2012 1:48 pm
by SleepyHead
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
Re: how to teach ratios... advice needed!
Posted: Fri Nov 16, 2012 1:56 pm
by IronMikeTyson
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
Re: how to teach ratios... advice needed!
Posted: Fri Nov 16, 2012 2:03 pm
by SleepyHead
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
Re: how to teach ratios... advice needed!
Posted: Fri Nov 16, 2012 2:23 pm
by Okanagan
In that case I'd probably do it per 25cm.
Re: how to teach ratios... advice needed!
Posted: Fri Nov 16, 2012 2:25 pm
by SleepyHead
thank you.