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Percentages

(6 Posts)
plum100 Mon 10-Mar-14 15:38:05

Hello, can anyone tell me how a year 5 child would be taught to work out simple percentages such as 30% of £2.20?

Dd says shes forgotten but i dont want to confuse her with different ways and terminology. Would she divide 2.20 by 100 then muliply the answer by 30?

Thanks in advance x

plum100 Mon 10-Mar-14 15:38:35

She shoul be working this out on paper by the way x

richmal Mon 10-Mar-14 15:54:39

Find 10% then multiply by 3.
So 22p * 3 =66p

TeenAndTween Mon 10-Mar-14 16:15:19

Agree with richmal . Find 10% by dividing by 10, then multiply by 3.

However the 'safest' way is to learn from first principles, which is as you said. Find 1% by dividing by 100, then multiply by 30.

Only if they can do the above then learn the shortcuts, and why they work.

After all, percent means by 100 or something, so useful to have the 100 in the calculation.

(Disclaimer - not a teacher)

plum100 Mon 10-Mar-14 16:36:56

Thank you x

PastSellByDate Tue 11-Mar-14 11:31:14

Plum 100:

agree with method suggested by richmal 10% then multiply by 3 - but the issue may be not understanding effect of multiplying/ dividing by tens (PLACE VALUE).

So first principles:

What happens to 2 when I multiply by 10 - it becomes 20 (In ye olden times - young dinosaurs like myself were told to write down the number you're multiplying 10/ 100/ 1000/ 10000000 by - then count the zeros in the 10/ 100/ 1000/ 10000000 and 'stick' them on the end) -

So once your DC gets

2 x 10 = 20
2 x 100 = 200
2 x 1000 = 2000
2 x 10,000 = 20,0000

you can then start the same game with numbers <0

2 x 0.1 (which is 10% but we'll get to that later) = 0.2
2 x 0.01 = 0.02
2 x 0.001 = 0.002
2 x 0.0001 = 0.0002
etc.... [the point being that for each 0 or 1 after the decimal you simply move the decimal over one place to the left on the number you're multiplying by - so 56 x 0.1 (moving left 1 decimal place on 56) = 5.6

Now per cent - literally translates from the Latin as parts per hundred.

So 10% is literally 10 parts per hundred and could be written as 10/100 (ten over 100) - which reduces down to 1/ 10 (you can divide numerator (top number) & denominator (bottom number - think 'd' for down) by 10 each to get 1/ 10).

1/10 (one tenth) can be written as a decimal as 0.1

ok - back to ye olde place value game

2.2 x 0.1 (moving number you're multiplying left one decimal place) = 0.22
(By the way you need the zero as a place marker in the units place - to make it clear that you're talking about a number smaller than zero).

Now you wanted to calculate 30% of £2.20

Up to you whether you want to think of 30% as

a percentage: 30%
a decimal: 0.30
a fraction 30/100 which reduces to 3/10

so there are a number of options - and it really is up to your child to determine which is easiest for them - but as richmal suggested:

30% = 3 x 10% - so work out 10% first and then multiply by 3:

10% of £2.20 = £0.22 (remember moving decimal one place to the left when dividing by 10/ multiplying by 0.1)

3 x £0.22 = £0.66

what I will say is that children often get muddled between working in pounds and pence. So teach them to check what they want the answer to be in and convert at the outset (so you don't forget).

----------------

2.20 x 0.30 =

Now this can look intimidating but let's make it easier

2.20 x 0.30 - first off ignore the decimals (but remember there are 2 decimals - that comes in handy later).

22 x 3 = 66

but there are 2 decimals - so we need to return two decimal places:

I can't draw it but basically if you imagine

66. - one hop left would be 6.6 and one more hop left would be 0.66

so £2.20 x 0.3 = £0.66 (or 66p)

---------------------------

finally you can handle this as a fraction

2.20 x 3/10 = 6.60/ 10 = 0.66

return the £ sign = £0.66

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I know (and boy have I been there) that it can be very worrying to teach your child your way or differently from the school - but please please remember that there is no ONE WAY to do maths. There are a number of ways to get to the solution - and knowing several (if well taught/ if the child understands what they're doing and the effect it has on the numbers their dealing with) can really be helpful. Once I started viewing this kind of thing as playing with numbers/ exploring patterns - it made the whole process of discussing my methods a lot more relaxing. My personal view is if the teacher is insisting your child MUST do it a particular way - and there clearly are a number of different methods available - the teacher most likely is very weak mathematically.

HTH

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