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Math's Help please - Simple rule to help teach Subtractions?

(15 Posts)
scotchbunny Sun 09-Oct-11 08:20:18

Hi

Looking for some advice on how to help DD(7) with her Math's, she has subtractions this weekend which she is finding tricky. I wondered if there was a specific rule that is taught or should be followed.

She has 36 sums to do, for example 93-5, 81-7 and so on. She basically counts backwards, which is ok for 52-3 but takes her an age with 22-9 which also leads to her becoming mixed up.

Any help appreciated! She struggles a bit with Maths so I would love to boost her confidence and give her some advice that makes the workings out a lot easier for her.

Thanks in advancesmile

well the school should be teaching her surely?
i can think of a couple of techniques but I am not a teacher

e.g. 22-9

9 is one away from 10 so she should take a whole ten off and then add the one back on. But I have no idea if that is a proper technique

Blueberties Sun 09-Oct-11 08:26:56

I agree with shriek. But I would say it sounds like she hasn't got her number bonds off pat - 1-9 2-8 3-7 etc etc so practice with those would be useful.

but shreik's method is what they used to teach via chunking (ridiculous becuae it's a mental maths method but anyway) and I think it's what many/most people do with swift mental arithmetic into adulthood.

PenelopePitstops Sun 09-Oct-11 08:37:37

like blue i would agree she needs much more work on her number bonds to 10. It is a huge advantage to be able to recognise instantly that 3 and 7 make 10. This then opens up a lot of formal and informal methods for addition and subtraction.

Sadly there is no simple rule for subtraction, and the method she is using so far shows she has understanding. Maintaining that understanding is more important than a quick fire method that she will mistake and easily forget.

Sheiks method is a good one to focus on, but relies on knowing number bonds and being confident with them. The sums you have written all break over the ten aswell, for example 52 - 3 is 2 down to 50 then take off another one. She could use this as a method but go with what she is happy with.

scotchbunny Sun 09-Oct-11 08:54:28

Thanks Ladies

She is ok with her number bonds, would recognise almost instantly that 3 and 7 make 10, is pretty good with number bonds up to 20 in fact. I think she is just slow with mental arithetic, still likes to count out on her fingers.

She is good with all her units of 10 so if that is a good technique to teach I will see if she gets it!

Thanks!

clutteredup Sun 09-Oct-11 09:01:54

Try using objects to hold and move around - maths is very abstract and the connection between the solid and the abstract isn't as clear to children as it is to adults ( well most adults) and sometimes using objects first can help her visualise things like taking off 10 then adding back one rather than introducing it as 'another method'. each method to us might seem obvious but it won't be obvious to her that doing that is the same as counting back 9 steps - if you have some counters preferably ones you can group into 10s (if stuck use 10ps and 1ps) then she can play by counting back or taking off 10 and adding back 1. I find 'holding and touching numbers' really helps them see what they are doing otherwise it is so abstract and not obvoius to them what they are actually doing-it's the understanding that's important.

School should have a policy for teaching calculations (they should give you a copy if you ask). At 7 I teach subtraction on a number line by counting on - start at the smallest number and count on to the biggest -

jump on number line to the next 10
add in 10s
jump to the unit number of the biggest number

ie

22 - 9

9+1 = 10
10+10= 20
20+2 = 22

So 1+10+2=`13

It is really hard without drawing a number line to show you but that is how I teach it, as per our calculations policy. School might teach it differently though. You can download number lines to practice on from sparklebox or similiar. Any help, feel free to pm. HTH

exoticfruits Sun 09-Oct-11 09:28:30

I would get a 100 square and then she could see that -9 = -10 and put one back or -11 = -10 and - one more. It all becomes clearer with the square (and they must use them at school.
Then 93-5 = -3 to 90 and take away another 2.
People are right-number bonds are the issue.

exoticfruits Sun 09-Oct-11 09:38:08

Good number bond games here

exoticfruits Sun 09-Oct-11 09:40:21

Great maths games (including subtraction) on Woodlands Junior School site and you can print off a hundred square.

scotchbunny Sun 09-Oct-11 09:50:49

Thanks guys, great advice, and thanks to exoticfruits for those fantastic looking linksmile

exoticfruits Sun 09-Oct-11 10:04:04

Always good to make it fun!

teacherwith2kids Sun 09-Oct-11 11:09:27

It really will depend on the school's calculation policy, as with taking away a much smaller number from a large number our calculation policy teaches counting back (first to the nearest 10, then however many are left over) whereas when the numbers are similar in size we teach counting on from the smaller one.

e.g. 81-7, I would teach:
How many back to the nearest 10? 1
How many left over to take away? 6 (as I've used up one of the 7 to get back to 80)
What is 80-6? 4 (using number bonds to 10)

However, if it was 81 - 77, I would teach:
These numbers are close together, so put them both on a number line.
How many from the smallest up to the nearest 10? 3 (from number bonds to 10)
How many more to get to 81? 1 more.
Answer = 3+1 = 4

We do a lot of preliminary work on ordering numbers, recognising whether numbers are close together on the number line or further away from each other etc to get children to recognise which method will be more efficient. The problem is if we ALWAYS teach counting up from the smaller number as sugarandspice does it becomes ludicrous for e.g. 222 - 5.

tutorsurrey Sun 09-Oct-11 11:55:06

Dear Scotch bunny and all,
I teach kids from the beginning, virtually, that subtraction is the same as ' what's the difference' and that you get the same answer by counting up our taking away.
Then I get them to say which method to use and the are very good at this, instinctively. My rule is that if smaller number is half of large number out doesn't matter which.
Use n lines and counters, avoid vertical columns.even just setting out vertically.and do plenty of practice with number bonds and differences of one digit.

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