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In not being able to do my 10 year olds math homework!

(147 Posts)
Livesforbedtime Wed 05-Mar-14 18:51:11


I don't understand this.. She has to work out 23 x 22..

Now I'm not the sharpest pencil in the box, but I have a degree and a responsible job... So WHY when I worked it out as:

20 x 20 = 400
2 x 3 = 6

Answer = 406

But it's not is it? It's 506.. But I don't understand why confused

My poor 10 year old DD is bewildered.. And I cannot understand why I'm wrong yet I know I am!

I suspect it have a mind block... Yes that's it.. I'm not truly stupid hmm ok I might be..

It doesn't make sense! angry

kim147 Sat 08-Mar-14 08:32:15

Message withdrawn at poster's request.

ShadowOfTheDay Sat 08-Mar-14 08:58:46

I like the way DD does long multiplication with her little grid...

she made mistakes at first, so we could explain why she was doing it wrong... e.g. kim... in your example sometimes she would forget it was 100 in the first box and put 1... and instead of 70 put 7 so she would end up with ridiculously small numbers and know she was wrong somewhere.

I think the "new fangled" grid method is a really neat way to explain how it all works ....

Jux Sat 08-Mar-14 11:33:42

I like the fact that there are lots of ways to do it now, rather than only having one way, so that a child can just do it whichever way they find works best for them. What bothers me is when greater emphasis is put on understanding what is happening than is put on how to do it. I think that first you just learn how, then you can worry about what/why/when. When dd was in primary, it was the other way about. If she had just learned tables (yes, by rote) she would have found maths easier all the way through (she's doing GCSEs now).

spottysocksandshoes Sat 08-Mar-14 23:16:23

I have no idea what's going on in this thread. I hope that brings you comfort OP grin

complexnumber Sun 09-Mar-14 09:19:05

The problem I have is trying to explain why, adding (say) 10% to a quantity, then subtracting 10% from your answer does not take you back to the original quantity.

I can show it with concrete examples, but I'm not sure I am convincing.

ShadowOfTheDay Sun 09-Mar-14 10:27:44

complexnumber - it is just how arithmetic works - 10% means ten out of every hundred.... of the number you are CURRENTLY using...

so adding 10% to your number makes it a different - bigger - number...

10% of THAT number will be bigger than your original 10% so it cannot take you back to your original number.

phantomnamechanger Sun 09-Mar-14 14:57:45

example - you have £1 and gain an extra 10%, so you now have £1.10

you deduct 10% OF THE NEW TOTAL, which is 11p, leaving 99p (as opposed to the original £1)

10% of any number is 10% of that precise number only and not equal to 10% of any other number. in the same way that half (50%) of a 100g bar of chocolate is not the same mass as half or 50% of a 200g bar.

There is a different way of approaching this situation which may be confusing you - you might be asked "I have x amount of money and earn 10% interest. I now have £1.10, what did I have in the first place. In this example you are working on the fact that the known sum is 110% of the original, so you need to divide by 11 not 10

LauraBridges Sun 09-Mar-14 15:30:56

There are often different ways to work out maths problems and it's good to know a few or at least have a means of double checking your answer to see if it's roughly right.

I don't help children with homework but I do remember a reasonable amount of maths as I quite liked it and was in the top set, got an A etc. It has a purity and logic to it which some other subjects don't.

marcopront Sun 09-Mar-14 15:43:08

I think complex number is a maths teacher and her question was not about why it works but explaining why it works. i agree with her it is one many students struggle with.

GarlicMarchHare Sun 09-Mar-14 15:56:24

YY, it's an odd one. Thinking back, I'd been doing it on paper since about 8 years old, but didn't get to grips with the practical fact of the matter until I started working in a shop. It is, of course, that the value has changed (to 110) so ten per cent of that is the second of two sums.

Still had to pause & work it through for several years!

kim147 Sun 09-Mar-14 15:58:19

Message withdrawn at poster's request.

anothernumberone Sun 09-Mar-14 16:02:43

The thing is percentages are a proportion of a single number so every single number has its own say 10% which only relates to that number and no other number so it reasonable to say that there should be no expectation of complex numbers scenario being the case.

complexnumber Sun 09-Mar-14 17:54:52

I think complex number is a maths teacher and her question was not about why it works but explaining why it works

Thank you Marco, for so many students it just seems counter-intuitive.

kim147 Sun 09-Mar-14 17:58:17

Message withdrawn at poster's request.

GarlicMarchHare Sun 09-Mar-14 21:33:27

Kim, you're doing the equivalent of saying "You must be bloody stupid if you don't understand it!" It may even be true, but it's still a remarkably unhelpful observation.

anothernumberone Sun 09-Mar-14 21:38:07

Garlic I don't think she was. I teach at 3rd level and we get plenty of students who do not know how to do percentages I don't think or call them stupid, I teach them how to do it they just have not learned properly yet.

kim147 Sun 09-Mar-14 21:42:34

Message withdrawn at poster's request.

kim147 Sun 09-Mar-14 21:47:52

Message withdrawn at poster's request.

GarlicMarchHare Mon 10-Mar-14 00:51:33

Perhaps I misunderstood, Kim. Thanks for replying.

sjnatnumeracy Mon 10-Mar-14 11:59:43

Message deleted by MNHQ. Here's a link to our Talk Guidelines.

Livesforbedtime Wed 12-Mar-14 21:49:21

Aha! Having spent 20 minutes with the DH tonight I can now officially do long multiplication!

Yes I'm probably far too old to have only just learnt but at least I did!

Actually quite proud of myself grin

GarlicMarchHare Wed 12-Mar-14 22:07:30

grin flowers Better late than never!

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