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Roz12

Posted: Sat Aug 31, 2013 8:42 pm 

Joined: Wed Nov 02, 2011 9:45 pm Posts: 118

Hi All,
Please could someone tell me how to teach following Q'ds to a ten year old in a simple and fast way?
Much appreciated.
1.66.6 times 12.44
2. Divide 74.7 by 16.6
3.Represent 99/400 as a percentage
Thanks


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mystery

Posted: Sun Sep 01, 2013 6:49 am 

Joined: Tue Jul 21, 2009 9:56 pm Posts: 8419

66.6x12.44
Multiply first number by 10 and second by 100 so you have two whole numbers to multiply together. Then divide the answer by 1000 at the end to get back to what it should have been.
74.7 div 16.6. Multiply both numbers by 10 and then do it. No need to adjust at the end because if you multiply or divide both numbers in a division calculation by the same number the answer remains the same.
99 div 400 as a percentage  multiply by 100


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leanmeamum

Posted: Sun Sep 01, 2013 8:39 am 

Joined: Sat Dec 03, 2011 3:14 pm Posts: 654

the easiest way to multiply decimals is to remove the decimal points and multiply them as normal multiplication sums and add the decimals afterwards.
66.6 times 12.44  you would multiply 666 X 1244 and add the decimal point 3 places starting from the right. The question has 3 numbers after the decimal point in total (1 after 66 and 2 after 12) therefore the answer should also have 3 numbers after the decimal point.
so if I had 0.00004 X 0.1, the answers would be 0.000004 (the question has 5 numbers and 1 number after the decimal points, respectively). I just multiply 4 and 1 and count up the number of numbers after the decimal point and add the appropriate number of zeros after my decimal point.
**************
With division of decimals, the divisor should always be converted to a whole number before dividing.
So 2.5 divided by 0.005 would work as follows 
Write them out as a fraction for ease of understanding
the divisor/denominator has to be multiplied by 1000 to make it a whole number (0.005 X 1000 = 5) whatever is done to the denominator has to be done to the numerator so 2.5 also has to be multiplied by a 1000 (2.5 X 1000 = 2500)
Finally, divide 2500 by 5 to give you 500 as the answer.
**********************
% are just special fractions of a 100
99 is the number out of 400 so write them out as a fraction and multiply by 100 to give you the %.
Another method is to use ratios
99 : 400 ? : 100
400 divided by 4 gives you 100 so we divide 99 by 4 to give you the % of 24.75 (whatever you do to the Righthand side has to be done to the lefthand side and vice versa)
*****************
Hope the explanations will help your child to understand these topics better. Decimals, fractions, ratios are used for a lot of other calculations as well so it is important to understand them really well.


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Guest55

Posted: Sun Sep 01, 2013 12:32 pm 

Joined: Mon Feb 12, 2007 1:21 pm Posts: 13039

The method suggested above i.e. counting decimal points is not a good method!
An estimate will quiclly tell you the magnitude of the answer
66.6 x 12.44 is roughly 70 x 10 i.e under 1000.
Where have these questions come from! They are far more suitable for a calculator ....


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Roz12

Posted: Sun Sep 01, 2013 1:02 pm 

Joined: Wed Nov 02, 2011 9:45 pm Posts: 118

Hi all,
Appreciate all the methods suggested. I will try them out on my daughter.
The questions are from a mock exam which my daughter failed miserably and I am very anxious if these come in the real thing she will not pass so I need to teach her all the methods.
Do questions like this not come up in real thing???
Thanks all.


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Guest55

Posted: Sun Sep 01, 2013 1:05 pm 

Joined: Mon Feb 12, 2007 1:21 pm Posts: 13039

Which exam? They are not KS2 appropriate questions.


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Roz12

Posted: Sun Sep 01, 2013 1:09 pm 

Joined: Wed Nov 02, 2011 9:45 pm Posts: 118

Hello Guest 55
I've pm'd you!


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mystery

Posted: Sun Sep 01, 2013 8:30 pm 

Joined: Tue Jul 21, 2009 9:56 pm Posts: 8419

I think it should be suitable to teach a ten year old who understands place value correctly how to multiply two decimal numbers together by multiplying them first by the correct number to get a whole number and the dividing by the correct number at the end, or by seeing what the correct order of magnitude for the answer should be for the answer and adjusting accordingly at the end.
It might not strictly be ks2, but goodness knows how one really defines that, but shouldn't be too difficult for a selective school child. We did it frequently at junior school.
Selection tests do not seem to be designed according to what is and is not ks2.


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parent2013

Posted: Mon Sep 02, 2013 2:12 pm 

Joined: Mon Aug 12, 2013 9:13 am Posts: 452

The decimal question is fairly straightforward  just multiply the numbers ignoring the decimals and then count/put the decimal place back into the answer.
Agree with mystery on KS2 syllabus which is just for reference however the tests are designed much tougher than that especially the ones for super selectives. I was shocked ot see that there's little or no algebra in KS2 and it is expected to solve equations by hit n trial which may take forever. There are tons of questions that I have seen which can be solved by algebra in seconds.
I suggest that the child should be extremely confident to convert decimals > percentage> fractions > back to decimals.


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leanmeamum

Posted: Tue Sep 03, 2013 7:06 pm 

Joined: Sat Dec 03, 2011 3:14 pm Posts: 654

There is a very hazy line between KS2 and the exam questions. The syllabus might be KS2 but the questions and understanding of the syllabus is definitely of a much higher degree.


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