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Manana

Posted: Tue Aug 02, 2011 11:03 am 

Joined: Wed Mar 09, 2011 2:22 pm Posts: 710

My mind has gone blank! Even just a simple amount like 'Find 0.8 of 16'?
I think it's too hot today!


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tco

Posted: Tue Aug 02, 2011 11:31 am 

Joined: Fri Nov 16, 2007 9:49 pm Posts: 37

I always use the take it down to 1% and then up to x%
ie divide by 100 then multiply by 80
in this case 16/100*80 = 16*8/10 = 128/10 = 12.8
when you ask a child what is 50% of something they divide by 2 which is easy (actually they are dividing by 100 and multiplying by 50) but when you ask what is 60% of something they get stuck
The opposite can be used when you have need to find what 100% is of something eg something costs £12 after a 40% reduction what was the original cost?
ie divide by the 60 (this is what % we have ie 10040) then multiply by 100 = 12/60*100 = 12/6*10 = 20


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moved

Posted: Tue Aug 02, 2011 11:39 am 

Joined: Fri Oct 12, 2007 12:42 pm Posts: 3826 Location: Chelmsford and pleased

I would go for the standard percentage method too, but ...
Other ways depending on the child.
Straight arithmetic: 0.8 x 16 is 0.8 x 8 x 2 = 6.4 x 2 = 12.8
Fractions and cancel (try to imagine two multiplied fractions are written below):
8 16  x  10 1
Cancel the 16 and 10 so left with 64 / 5.


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no_ball

Posted: Tue Aug 02, 2011 12:21 pm 


i taught my DS to put a 1 under the decimal point and then zeros after the numbers to the right of the decimal point to make the whole thing a fraction. The rest is simple division.


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svg123

Posted: Tue Aug 02, 2011 12:57 pm 

Joined: Thu Feb 24, 2011 12:34 pm Posts: 210

I am sure your DS knows that 0.8 is also called eighttenth (ie. eight out of 10) .. so, 0.8 of 16 will be: 8/10 x 16 OR 8/10 x 16/1= 128/10 = 12.8


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no_ball

Posted: Tue Aug 02, 2011 1:02 pm 



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mystery

Posted: Thu Aug 04, 2011 2:48 pm 

Joined: Tue Jul 21, 2009 9:56 pm Posts: 8321

Get a good primary maths book and get him/ her to work through all the different fractions and percentage exercises. Then see if they can do that question without your help.


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Fismum

Posted: Thu Aug 11, 2011 12:34 pm 

Joined: Tue Sep 07, 2010 10:32 am Posts: 35

the best way i explained to my DD was to first forget about the decimal point and just multipy straight away i.e 16X8 = 128 then now go back to the figure with decimal point which is 0.8, count from back how many figure before the decimal point, it is just one now go back to the answer, count from back too and insert the point after one figure 12.8. that's it.
Usborne junior maths illustrated also used this method of forgetting the decimal point first of all and multiplying straight away.
hope this helps


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Guest55

Posted: Thu Aug 11, 2011 1:09 pm 

Joined: Mon Feb 12, 2007 1:21 pm Posts: 12650

These 'methods' of working out where the dp should be are not advised. Much better to estimate the answer and use that 
Last edited by Guest55 on Thu Aug 11, 2011 7:39 pm, edited 1 time in total.


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menagerie

Posted: Thu Aug 11, 2011 7:26 pm 

Joined: Thu May 26, 2011 8:37 pm Posts: 577

Fismum, that's exactly how we were taught to do it at school. Take the decimal out to multiply then put it back in at the end. But it is easy to slip up with that method. (I always did and ended up very confused.)
svg123's answer seemed clearest to me (speaking as someone with no natural aptitude for maths.)


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