Daft question - how are our dc taught to do subtraction?!

(34 Posts)
IwishIwasmoreorganised Fri 05-Oct-12 21:00:49

I can obviously do it, but have poor skills at explaining my way to ds!

He's come home with his first subtraction homework - numbers up to 100 and also a number square to use (10 rows of 10).

I'm sure that he's done some at school but tonight's quick look and ask resulted in a stroppy, "how am I supposed to know?!"

Please can some kindly soul explain how I should explain this to him. I'd be very grateful.

sausagesandwich34 Fri 05-Oct-12 21:04:31

my dcs started with number lines and chunking

and now they have moved on to the more complex collumn method which looks like this...

23
- 25
__

which is how I was taught to do it and is far simpler than chunking

sausagesandwich34 Fri 05-Oct-12 21:05:10

well, not exactly like that -looked right when I was tying in hmm

Brycie Fri 05-Oct-12 21:09:09

Chunking is hell. We did ten to the top, one to the bottom. Carrying over is a good way. I think they focus on chunking now, which is basically mental arithmetic written down. For speed and accuracy, carrying over is best. Sometimes they teach Russian, Chinese, Arabic methods of arithmetic, and tell the child to choose one to use. Tremendously helpful as you can probably imagine. Cue lost generation that can't add 50 and 50 without a calculator.

Brycie Fri 05-Oct-12 21:10:20

If you want to help I would work on number bonds up to ten and twenty, and then a hundred (ie how it mirrors the tens) - forwards, backwards, upside down.

BlueberryHill Fri 05-Oct-12 21:10:29

Shit, we use bricks and number lines......

And I'm an accountant......

Brycie Fri 05-Oct-12 21:11:59

If only everyone had the maths skills to become an accountant.

IwishIwasmoreorganised Fri 05-Oct-12 21:14:14

I'm lost!

I could happily help him using the column method but it's clear that that isn't how they're learning it at this stage.

Can you explain more about chunking in very simple, non teachery language for my tired old head please.

Brycie Fri 05-Oct-12 21:18:11

So far as this tired old head can remember it, I think you break the numbers into chunks, just like in mental arithmetic. So if you are subtracting 27 you subtract twenty, then ten, then add three. I think. I might be wrong. My children used carry over to check their answers from chunking. Carry over takes seconds. Chunking can take 'king ages.

Brycie Fri 05-Oct-12 21:20:23

I think you end up with something that looks like this.
63 - 27

63
- 20
-
43
- 10
-
33
+ 3
----
37

Am really happy to be told by a maths teacher I'm wrong, I won't mind at all. But at least we are keeping it alive unitil one comes along.

IwishIwasmoreorganised Fri 05-Oct-12 21:24:54

Blimey!

It makes sense but is awfully long winded!

Happy to do it that way with him if he seems to recognise the technique.

I'm hoping that he's more receptive to having a go at it in the morning and just sits down and rattles them all off. That's how the additions have been going.

BlueberryHill Fri 05-Oct-12 21:25:00

Brycie, I see it as a very creative profession

Brycie Fri 05-Oct-12 21:26:47

Yes I could do with a creative accountant myself grin
Iwish - I might yet be wrong. Don't take that as gospel, I hate chunking with a passion!

IwishIwasmoreorganised Fri 05-Oct-12 21:35:26

Not to worry Brycie if it's not how they've been doing it. At least it gives me somewhere to start.

Thanks for your help.

LeeCoakley Fri 05-Oct-12 22:25:21

How old is he? If he's KS1 and just been sent home with a 100 square then it could just be practise on how to use it. E.g. 54 - 23. Find 54, go up 2 rows (because that is 2 lots of 10) and then go 3 units to the left. The answer is under your finger!

IwishIwasmoreorganised Fri 05-Oct-12 22:29:42

He's 6, in year 1

theweekendisnear Fri 05-Oct-12 22:38:06

If the question is 63-27, I would try with

1) 63-20 = 43
then split the 7 into 3 and 4
43 -3 = 40, but I still have to take away 4
40 - 4 = 36

or

2) "what if" method
what if instead of having 63-27 I had 63 -30, then the answer would be 33.
But I took away 3 too many (30 is bigger than 27 by 3), so I have to add 3 to 33, which gives me 36

or

3) "what if" method number 2
what if instead of having 63-27 I had 67 - 27, then the answer would be 40, but I started with 4 too many (67 is bigger than 63 by four), so I have to take 4 away from 40 = 36

However, I think this is a hard one to do in your head. Probably he has to do it looking at the 100 square, so he has to do "take away 10" to move vertically, take away one to move left.

Another thing you could do is having 63 beads, or pasta pieces, in 6 groups of 10 and then the three units, and then he can count 63 .... 53.... 43... while he takes away 10 at a time, and then counting 7 back from 43, to get to 36.

Show him in many different ways, I think the exposure to different ways is very important in Maths. Also, you could try the cuisinnaire rods.

IwishIwasmoreorganised Fri 05-Oct-12 22:44:52

Yes. Lots of different ways to get your head around maths is good I agree and certainly helped me when I was younger.

Thanks for those suggestions, your way of explaining things is great and much clearer than I think I'd have come up with!

Off to google cuisinnaire rods!

LeeCoakley Fri 05-Oct-12 22:52:55

If he's year 1 and not genius level then it will be just going up the rows and back along the columns as I suggested. Our lower ability year 2s are consolidating this at the moment.

Finbert Fri 05-Oct-12 22:58:42

Or a backwards number line where you count on in chunks. E.g 67-23 put 23 at start of number line 67 at end then jump in whatever increments they are comfortable (if new to method 10's, 5's, 2's or 1's) if more confident maybe 23+7= 30 + 30 = 60 + 7 = 67 then count up all numbers + so 7 + 30 + 7 = 44

IwishIwasmoreorganised Fri 05-Oct-12 23:04:41

Thanks. He's not daft, but I wouldn't say he's a genius!

We'll go for the up the columns and along the rows of the number square first and see how we get on.

It's interesting and reassuring to see how many different ways are being used out there!

LeeCoakley Fri 05-Oct-12 23:07:12

Sorry - I hope I didn't sound rude about the genius comment!

IwishIwasmoreorganised Sat 06-Oct-12 17:37:55

Not at all.

Not sure how i'd cope if i had a genius on my hands!

JumpingJetFlash Sat 06-Oct-12 20:24:25

I would use counting on from the smaller number on a numberline.
Eg 63-27
27 at one end, 63 at other
+7 = 30, +30= 60, +7= 67. So answer is 44

Can use the hundred square to work how many units to next ten, how many tens to nearest 10 without going over, add rest of units

Chunking is division terminology really

Sorry can't draw to make it clearer

JumpingJetFlash Sat 06-Oct-12 20:27:52

Crap realised its add 3 to 30 and add 3 to 63 making answer 36 :-/ That'll teach me not to check my post :-/

63 - 27 = 36

27+3=30+30=60+3=63 but on a numberline

Sorry again

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