Adding larger numbers - help needed

[Honeybarbara]

Sorry for being dim, I am hoping to find out the current method taught in schools for adding and subtracting larger numbers. I know you add the tens first, then the units, but how do they do it if the units add up to 10 or more eg: 27+27?

Obviously the way I would do it is to set it out in columns, but how would you do it in your head? Are they supposed to be able to do this mentally in Y3?

Thanks for you help.

[GreatBallsOfFluff]

Hi I'm not sure if this is still the way it's taught, but I remember doing the units first e.g. 7+7=14
So the last number of the answer will be 4, then add the tens with the 1 carried over e.g. 20+20=40 then plus the 10=50

As I say, am not sure whether this is still the way they do it and also I can't help with whether they are expected to do it in their heads in Y3.

[thecloudhopper]

Im not sure if at year three they are supposed to add in their head big numbers but the way I do it in my head for 27 + 27 I would look at 20 add 20 then 7 and 7 then add the two numbers.

[Honeybarbara]

Thanks guys, that makes sense. I would still be interested in a definitive answer if there are any primary teachers out there, don't want to muddle the dcs any more than necessary!

[smee]

Ask the school - honestly they should want to tell you, especially if sums are part of homework. We get sheets every half term for parents saying what's being taught in each subject area and how they're teaching it so we don't confuse them by showing them a different way.

[swill72]

For mental methods, you add the most significant digit first (the largest). So if you're doing 47 + 36, you do 40 + 30, then 7 + 6, then 70 + 13. Obviously, they then have to learn to add units first when they do 'formal' written column addition, but they do so much mental addition before moving on to written methods that it's well embedded. Also, the first 'informal' written methods would be written down in the way I described for mental methods. Hope this helps (I'm a teacher by the way!).

[cece]

partitioning.

27 becomes 20 + 7

so 27 + 27 becomes

20 + 20 = 40

7 + 7 = 14

therfore

40 + 14 = 54

That is one of the ways they would do it in Y3

[hbfac]

Honey - i know you're asking about current methods in schools but I'm going to pass on what I do at home, just because it's a fun alternative.

Warning: contains sweets.

Have three different kinds of sweets. Eg. buttons for units; gumdrops for tens; marshmallow for hundreds.

23 + 18 = 3 x buttons and 2 x gumdrops + 8 x buttons and 1 x gumdrop.

Arrange like a sum.

Add the buttons, swap 10 x buttons for a gum drop. Add the buttons.

Answer 1 x button and 4 x gumdrops.

Just in case anyone finds it helpful.

Or maybe just wants an insight into the activities of parents desperate to reinforce maths at home!

Not necessary to eat all the sweets. But the odd unit going missing is not commented on in this house.

[Hulababy]

DD is in Y3 and wasn't confident doing the 20+20+7+7 thing, so I showed her how to do it in columns, carrying units over into tens column. She got this straight away and is now happy to ue this to add any numbers, regardless of size. Her teacher was fine, so long as she had a way that worked for her.

In Y3 they aren't expected to be able to do this in their heads yet, without working.

[mussyhillmum]

Many thanks for this thread - my DC's school is weirdly reluctant to explain the new maths to parents. Thanks to you lot, I now understand how to add larger numbers. Could someone (swill72,perhaps?) please explain how you subtract larger numbers like this 47 - 29 = ?

Mrs Magnolia - thanks for the book reference - another one for the bookshelf methinks!

[BigTillyMint]

Google Primary Strategy for the curriculum they will be covering - it will have examples of different ways of doing it.

The site is notoriously difficult to navigate though

[Hulababy]

Not sure on official way of subtracting, but DD does it this way:

47-29

Look at lowest number (29)

Jump to next 10 number, using number bond info (30) = 1

Jump to the nearest 10 to the biggest number (40) = 10

Jump to the unit part of biggest number (47) = 7

So, 1+10+7 = 18

This does make sense when written down properly, promise. You can draw the jups on a number line to illustrate it too.

[Hulababy]

DD also knows how to do subtraction through columns, with carrying - but found the method below easier to understand, and prefers to use this at present.

[humptynumpty]

Honey, I am teachery type!!!
At the moment, it is not encouraged to do sums like we used to in the old days. They want to teach kids loads of different ways of looking at sums and then being able to pick one which works for them.
In year 3, I think an average kid would be pretty much able to do that in their head e.g.
27 + 27 =
20 +7 + 20 + 7=
40+14=
54

Other ways of looking at it would be if you know 25 + 25 = 50 then add on the extra 2 and 2 to make 27+27 iyswim

Also the way hullababy says is v popular

Finally, on a 100 square start at the bigger number and add on the smaller one e.g. 56 + 23
Put your finger on 56. jump down 2 rows to add 20 (makes 76) then jump to the right 3 to end up on 79.

Hope that helps, can try to find some links if you like!

[humptynumpty]

sorry honey forgot to add, that when teaching adding larger numbers, the teacher would deliberately start by adding numbers that didn't go over ten eg 27+21 so the units don't make more than 9 iyswim to make it less confusing, that way when you break down the numbers into tens and units you end up with a tens and units to put back together e.g. 27+21 --> 20+20 =40 and 7+1=8 then 40+8=48

[IAmTheEasterBunny]

I would expect average+ children to be able to do that in their heads in y3, probably using partitioning (adding tens then adding units, then adding both) OR (If they can imagine a number line (which they should have done to death in Y2) as:

27 + 20 = 47 (they should all be able to count on in tens easily)

47 + 7 = 54

[Honeybarbara]

Love the sweets idea, must try immediately!

I am a visual learner myself, as is dd. I'm sure she will be better at it once she can do it in written form.

Thanks for the tips.

[swill72]

Whoever mentioned the number line, this is probably the most favoured method and involves counting on from the lower number.

47-29

29 + 1 = 30. 30 + 10 = 40. 40 + 7 = 47.
The answer is the numbers they've added on (1 + 10 + 7 = 18)
They start using number lines very early in KS1, so should be easy to use. It's a very visual method - hard to show on here!

Or:
47 - 30 (29 +1) = 17 Add the 1 back on = 18

Or:
47 - 20 = 27. Recognise that 9 is 7 + 2, then put that into practice with 27 - 7 = 20, then 20 - 2 = 18.

I should mention that the most important thing is for them to understand the relationship between addition and subtraction(i.e. be able to recognise that 6+4=10, 4+6=10, 10-6=4, 10-4=6 are all connected).

Also, knowing their number bonds and doubles/halves is a massive help!

And ... remember many mental methods are pretty much halfway between the two and may need quick jottings if they're unable to hold the numbers in their head.

Hope that helps a bit.

[swill72]

By the way, the sweeties idea - great, really creative! (Looking forward to some easier sweeties maths with 5 yr old DS!) Make sure your DD is happy with decomposition before you do this though as the whole substitution thing may throw her if she's not ready!

[IAmTheEasterBunny]

Not forgetting number bonds to 20, to make subtraction easier through the tens numbers i.e. 73-8 :
If the child knows 13-8=5, s/he can mentally check the answer, as s/he will know it has 5 units.

They might not make this link in Y3 but it's something to aim for.