# Talk

## Maths help, simple subtraction but the best way of explaining the answer.

(27 Posts)
MotleyCroup Mon 31-Mar-14 19:47:37

Ds (Y2) came home with some subtraction homework.

Tbh I'm finding it a struggle to explain the best way to get to the answer.

So you basically had to find the hidden number.

_ - 23 = 14 (I explained the best way was to add the answer to the remaining number?) so the answer in the case was 37.

36 - _ = 24 - we know the answer is 12 by subtracting 36-24 but how is the best way of explaining this?

I know there is a longer way around by using tens and units.

Think I need to go back to school.

It's the explaining how we get to the answer which I'm really pants at

Also a simple subtraction such as 26-12
I'd set this out on paper the way I was taught
So 26
12
=14

Would Y2 set it out in this way?

Sorry I'm a complete dinosaur!

FunkyBoldRibena Mon 31-Mar-14 19:54:28

Try it with pennies/buttons/chocolate chips - anything he can count out

37 pennies minus [what] leaves 24 on the table. [what] is the answer.

joanofarchitrave Mon 31-Mar-14 19:57:29

I would try using a number line at this stage. I'm a dinosaur too so had never heard of them, but they do make it simple.
Website to show you what they are but I would just draw one out on paper, much more likely to make sense.

MotleyCroup Mon 31-Mar-14 19:59:24

Also the best way of 'carrying over' numbers in subtraction.

Example

28
- 9

So I'd cross the 2 out carrying the 1 over to the 8 etc

How is this set out and explained in a y2 class?

MotleyCroup Mon 31-Mar-14 20:02:35

Also the best way of 'carrying over' numbers in subtraction.

Example

28
- 9

So I'd cross the 2 out carrying the 1 over to the 8 etc

How is this set out and explained in a y2 class?

MotleyCroup Mon 31-Mar-14 20:05:15

Whoops sorry, wifi had a bit of a moment!

Thanks Joan and Funky.

Maths has come back to haunt me!

Littleroobe Mon 31-Mar-14 20:05:29

The second one you can do on a number line (well both u can but 2 is easier to see)
36 at the start 24 the other and found how many in between. By counting on or back whichever is easier for them.
First one 14 on the right and count back adding on 23. The number you end at is the answer.

Then check it by doing the question with the number you have found. That's what I'd do anyway.

Tho to be fair this is a difficult concept for children to see, especially when you are breaking number barriers.

Littleroobe Mon 31-Mar-14 20:08:09

Number lines.
1. Put 14 on the right hand side and count right to left adding on 23. Number you end on is the answer. Then do another number line to check the calculation as it is written

2. 36 on left of number line 24 on right. Then count how many in between. Again once you have the answer draw another number line to check using the question as written.

Tho to be fair this is a difficult concept to many children.

Littleroobe Mon 31-Mar-14 20:08:56

Oops double post thought it had not posted! Tho second one probably reads easier!

TheBuskersDog Mon 31-Mar-14 20:09:26

Ask your son how they are doing them in class, probably on a number line.

I don't even understand what you are talking about with 'carrying over', don't whatever you do try and teach him column method.

TeenAndTween Mon 31-Mar-14 20:17:18

Agree with others, number line all the way.

MotleyCroup Mon 31-Mar-14 20:34:22

He's recently changed schools and says that they occasionally used a number line at his old school but couldn't use one when the did 'big maths'?

He tells me that they don't use a number line at his new school. I think I'll possibly have to have a chat with the teacher to ask their methods.

He worked out the easier subtractions, just the missing numbers he guessed at rather than trying to work out.

staticdust Mon 31-Mar-14 20:46:09

I think you you have explained it in a good way OP and also you showed your ds a method of doing subtraction, which will be useful to him in the coming maths/years.
Out of interest, Why not column subtraction?
DC is in y3 and the only way they are adding and subtracting is in column, small class, but all of them.
Had a parent meeting on Friday, and flicked trough DC maths book, addition and subtraction three digit and four all done in column.

staticdust Mon 31-Mar-14 20:47:17

Three and four digit, duh.

Mon 31-Mar-14 20:58:34

I think I'd explain that subtraction can work two ways so that x - y = z means that x - z = y too. You can show this easily with physical objects.

As for laying it out on the page, I don't know how they do it in schools these days so can't help!

MotleyCroup Mon 31-Mar-14 21:11:00

Thank you Static and 2b.

I would love this subject to come easy to me but it never has and I'm determined not to pass this on to ds.

I'm as confused as a confused person can be.

I think the physical route of explaining will be easier. Have to say I did wonder why the column method isn't used

staticdust Mon 31-Mar-14 21:31:02

As far as I know column method is being used, providing children are confident with place value and have grounding what each digit represents,
I do believe number line has it's place while the child is growing in that confidence regarding place value, but at some point children should move onto traditional methods, I also have heard, teachers correct me if I am wrong, new maths curriculum is calling for more traditional methods.

PastSellByDate Tue 01-Apr-14 14:06:13

In response to 2bearnot's Mon 31-Mar-14 20:58:34 post above:

I think I'd explain that subtraction can work two ways so that x - y = z means that x - z = y too. You can show this easily with physical objects.

As for laying it out on the page, I don't know how they do it in schools these days so can't help!

The question is what take away 23 = 14 (___ - 23 = 14 or could be written as x - 23 = 14) and the underlying principle they want your child to understand is that by adding 23 to both sides of this equation you can solve x.

What take away 23 = 14 is in fact the same thing as saying

What = 23 + 14 (because x - 23 + 23 = 14 + 23 which reduces to x = 14 + 23).

So how to explain this to a child - I agree use objects - but also use smaller numbers at first. Let's just use 10. So ask your child to make two piles (any number) from ten objects.

What take away pile 1 = pile 2

(well in this case we know the answer is 10 because we've done this) - but let's imagine we didn't know that. How could we figure it out.

You have a big number and take away some of it to get a smaller number.

So grab a piece of bread/ an orange/ a chocolate bar.

You have a whole piece of bread - you take away a big chunk and have part of a piece of bread left. Imagine if you could reverse that and make the whole piece of bread - what would you do? Stick the chunk onto the bread with the bit missing. So it's the same thing in math.

What big number - 23 = 14

(stick the chunk on the bit of bread left over)

What big number = 23 + 14.

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

Now I'm presuming your DC can do the maths to solve this either by partitioning (adding 20 + 10 to get 30 and 3 + 4 to get 7 and then getting 37/ or in jumps - so adding 10 to 23 to get 33 and then 4 to 33 to get 37.)

HTH

MotleyCroup Tue 01-Apr-14 15:40:57

Thank you PSBD!

Have to say this stumped me with regards to the way to explain (it doesn't take much).

Is this a typical Y2 maths exercise?

Tue 01-Apr-14 15:51:49

DD1 is in Year 2 and one of their homework sheets always contains a group of sums. They call it a Factor Family:

e.g. 12 + 7 = _ then 3 blank boxes.

Dd1 has to fill 19 into the _ above and then write the following (one per blank box)
7 + 12 = 19
19 - 12 = 7
19 - 7 = 12

which helps them to show that _ - 12 = 7 can be worked out from 12 + 7 = _

Hope that makes sense. It's far more difficult to type into an MN post than it is to see on paper!

MotleyCroup Wed 02-Apr-14 10:18:36

Crafty, that's a great way of 'seeing' the link between the numbers.

Beehatch Wed 02-Apr-14 21:33:45

My Y2 DD has been taught something similar to crafty, they call them number families so they often have to list 4 sums from a given fact. They do the same for multiplication and division, which has really helped her understand the concept of dividing.

PastSellByDate Thu 03-Apr-14 11:22:44

Totally agree with crafty approach (which for my DDs is taught as 'number facts')

I would just add one more to Craftynclothy's list of:

7 + 12 = 19
19 - 12 = 7
19 - 7 = 12

I would also include

12 + 7 = 19 (this is the commutative principle that in addition it doesn't matter whether you put 12 first or last in 12 + 7 or 7 +12 either way they both = 19).

So DD's learned four facts in total.

MotleyCroup Thu 03-Apr-14 12:32:30

Thanks PSBD.

It makes much more sense when the facts are set out in this way and also easier for me to help ds.

I presume he's done this at his old school but we've only ever had basic maths homework, nothing like this until he started at his new school.

It would have helped if there had been a system to the questions too but they were pretty random.

Thu 03-Apr-14 13:20:39

PSBD Dd1's has all 4 that you mention. Sorry, it's not very clear from my post. They give part of the first one so that all the children are doing the same group iyswim. So the sheet has 12 + 7 = printed in one box, she has to answer that and then work out the other 3 facts from that fact. Have I made it clearer or just confused it even more?!?

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