When you carry over from the next column...

[hobbgoblin]

In subtraction with 3 digit sums, and the sum is such that you have to borrow for the first column and then again from the second, except the second (tens) column is a 0 so ut needs to be rounded up to a ten before you can borrow and then becomes a 9.

How the chuffing norah do you explain that? I can't even do it for the benefit of you adults.

Help!!!!!!!!

[hobbgoblin]

Is it my explanation that is preventing all the wise replies? Perchance, like.

[WilfSell]

WEll

203 -
154

You're not moving a 1 over next to the three, you're moving a 10 over (actually) to make it 13

If you break it down into a grid (isn't that what they're taught to do?) of 100s 10s and single units, you can then logically 'get' it a bit more clearly.

So, you can't take 4 away from 3, so instead you're taking 4 away from 13+190

Or more correctly, 4 away from 13, then 150 away from 190...

The carrying a 1 is a quick technique but actually it is a 10...

Does this help?

[hobbgoblin]

It does and thank you. My only concern about my explanation of this is that DD's brain, being much like mine, will result in her making the 3 in your example sum 103 iyswim.

My mother has passed on a faulty mathematical ability gene which I have clearly propagated.

[Littlefish]

How old is dd? Is this her homework, and have you been asked specifically to do it in the column method?

[primarymum]

I would probably say that I cant take 4 away from 3 so need to use a ten from the next column. I have 20 tens (using the hundreds as tens) So if I use one to make the 3 into 13, I would have 19 from which I need to take the 15.
If she struggles recognising the number as 13 rather than 103, I would write it down as 10+3 first, rather than straight to 13.She should recognise this as 13, otherwise she doesn't have sufficient place value knowledge to be doing column addition in the first place!

[zazizoma]

I used to teach maths, and the method that worked best for me was to turn it into a story where each column represented a person or character that only had denominations of money in 1, 10, 100, etc. Then the story goes how one person borrows from the next, who borrows from the next, etc.

[Littlefish]

Primary mum - the understanding of place value is what I was sort of hinting at.

I'm a ks1/foundation teacher so I'm not sure in which year column addition/subtraction is taught, but I do know that children need to have a really secure understanding of place value and size of numbers before this method is taught.

It wasn't until I was 30, and an NQT that I realised that I was adding 10 and not 1!

[piscesmoon]

How old is she? She shouldn't really be attempting it without a secure grasp of place value, and using other methods to find the difference. She isn't 'borrowing' anything-it is exchanging.

[zazizoma]

Now that's interesting piscesmoon, how is taking a 100 from the 100's column and moving it to the 10's column viewed as 'exchanging'?

[piscesmoon]

It is difficult to write down in an understandable way. I was taught when I was at school that you borrowed a 10 and paid it back but although it works beautifully, it doesn't make sense!
Split the number into hundreds, tens and units (H T & U)
You exchange the 1hundred for 10tens (10x10=100), therefore where you had 203 you now have 100 and 10 tens so that you can take one ten and exchange it for units so that you have 10 units + the 3 units that were there already. You then have 1 hundred, 90 and 13
and you can take away 1 hundred and 50 and 4 and get left with 49.
Unfortunately I don't have the expertise to write it in the columns on the computer.

[zazizoma]

piscesmoon, I think I understand what you are saying. So it's not an exchange between columns, rather an exchange between 'currencies' in any given column. It is still borrowing between columns though, no?

[piscesmoon]

It isn't borrowing because you don't give it back. I remember my teacher saying anything borrowed much be given back while doing it, and we borrowed from the tens at the top and gave it back at the bottom. It works beautifully and you can do rows of sums with ticks and no understanding. I think it was easier in my day because everything wasn't in 10s and the penny dropped with me when you 'borrowed' 12 for a foot and 3 for a yard. Nowadays you cross through the top number and take it off at the top and don't have to 'pay back' at the bottom. This is easier to understand but not to write in a message!

[zazizoma]

I follow, and agree piscesmoon. It doesn't ever get "paid back." Should we use "steal" instead?

[piscesmoon]

Steal would fit the bill

[primarymum]

We generally don't teach column addition until Yr 5, although some yr 4's seem to work it out on their own!

[primarymum]

I meant subtraction!

[hobbgoblin]

Hi

Back to this after a night off the ole mumsnet!

Thank you for the varied replies. What seems consistent is the advice about her grasp of place value.

Maybe I could do some work on this with her at home just to see if she was being stretched overly with this homework.

She is Year 5 but behind on Math, again very much as I was at school.

I can teach KS1 maths - not brilliantly - but KS2 is beyond my capabilities.

[Littlefish]

That sounds sensible hobbgoblin. She needs to be really secure on what each digit means - ie. is it a thousand, hundred, ten or unit. Perhaps you could have a chat with her teacher.