# How did you teach subtracting from teens?

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For example: 15-7

8 yo is struggling with subtraction. She does fine with minuends 10 and below, but freezes when we get to the teens. Counting back or up more than 3 usually means her answer is off by one or two. My goal for the next week or two is to have her subtracting these numbers with ease.

So, what's worked with your kids?

She refuses to use C-rods, counters or other manipulatives because she says they're for babies. I might just force her to use them anyway, but I'd really rather not turn this into a battleground.

Thanks

ETA: We do have Right Start games and an abacus, so that's always an option.

Edited by shinyhappypeople
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with 15-7 you recognize that you do not have enough one's to subtract 7, so you subtract the 7 from the 10. 10-7=3. Then you add the 3 and the 5 and the answer is 8.

For small numbers this might seem a bit cumbersome, but it is perfectly scalable, so if your child can do 15-7, she can also do 35-7 or 65-27 or 350-70 etc.

BTW, I'm assuming she can already do 16-3, because that is essentially the same as 6-3, because nothing happens to the 10.

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with 15-7 you recognize that you do not have enough one's to subtract 7, so you subtract the 7 from the 10. 10-7=3. Then you add the 3 and the 5 and the answer is 8.

For small numbers this might seem a bit cumbersome, but it is perfectly scalable, so if your child can do 15-7, she can also do 35-7 or 65-27 or 350-70 etc.

BTW, I'm assuming she can already do 16-3, because that is essentially the same as 6-3, because nothing happens to the 10.

:iagree: We switched to Math in Focus (Singapore style math) at the end of first grade. Dd could not do these types of problems either. She would freeze up. Math in Focus taught her to draw two lines from the number fifteen off to the side. At the end of the lines you would break fifteen into it's two parts, 10 and 5. Then subtract the bottom number from 10. Take your answer and add it back to the 5. That is your answer.

It took dd a couple of weeks to understand the process. But now she can work a page of these problems as fast as I can without the lines or numbers anymore. And even using bigger numbers, like the pp pointed out. She just does it all in her head.

:lol:

Tara

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Thanks! I'm going to give it a try tomorrow.

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Does she know "doubles?" For example, 8+8=16? 7+7=14? For 15-7 you would see that you are off from your double for 7 by 1, so the answer must be 1 higher than 7. I wouldn't use it as the only method, because all the different ways to break up 10s and other numbers are very useful. I do think these number relationships are a handy and easily remembered tool as long as the student understands why they work.

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An actual useful answer from m would be the way RightStart Math teaches it.

15 - 7: first subtract to 10, and then subtract what's left. So first you do 15-5 (to get to 10) and then you still have 2 of your 7 left over. Subtract 2 from 10 to get 8. In order to do this kids have to know how to break numbers into their component parts.

It sounds more complicated typing it out than it really is.

Tara

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I love the responses already given and I agree with the way the SM teaches it. I know she doesn't like manipulatives, but base ten blocks are really fun and not for babies! She can trade the 10 in her fifteen for 10 ones and really see it that way. I think it is so important to teach composing and decomposing 10's when you are doing math under 20. That way, when you get to much more difficult problems, you already know how to do it. Have you read Liping Ma's book? It really is amazing and would be really helpful for you. Our library didn't have it, but they surprised me and ordered it when I requested it.

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An actual useful answer from m would be the way RightStart Math teaches it.

15 - 7: first subtract to 10, and then subtract what's left. So first you do 15-5 (to get to 10) and then you still have 2 of your 7 left over. Subtract 2 from 10 to get 8. In order to do this kids have to know how to break numbers into their component parts.

It sounds more complicated typing it out than it really is.

Tara

:iagree: That's what we do here - get to ten and see what's left over.

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An actual useful answer from m would be the way RightStart Math teaches it.

15 - 7: first subtract to 10, and then subtract what's left. So first you do 15-5 (to get to 10) and then you still have 2 of your 7 left over. Subtract 2 from 10 to get 8. In order to do this kids have to know how to break numbers into their component parts.

It sounds more complicated typing it out than it really is.

Tara

:iagree: We use MM and it also teaches to go to 10.

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An actual useful answer from m would be the way RightStart Math teaches it.

15 - 7: first subtract to 10, and then subtract what's left. So first you do 15-5 (to get to 10) and then you still have 2 of your 7 left over. Subtract 2 from 10 to get 8. In order to do this kids have to know how to break numbers into their component parts.

I believe this is the simplest method, and it's what I've taught DS6 (we use Singapore Math now, but started with 2a so it never came up). I also agree that learning the doubles is very helpful to memorizing these math facts. The Singapore Math method as stated may be good for easy and quick calculation, as it can rely on knowing the facts for subtraction from 10, but it's not the most intuitive and it's also not the best for setting a child up to learn borrowing. IMHO. :)
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I would not go another step until I knew she had her addition from 1-20 locked down solid. If the subtraction is shaky, make sure the addition is solid before going any further. Knowing how to add beyond 1-9 will help with that subtraction. If they know the method of 'getting to 10' with a question of '8+4' then the subtraction will be smoother.

You might consider looking at MEP year 1 for some basic practice with addition and subtraction within 20. Year 1 is spend working in myriad ways getting 1-20 solid.

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Dimes, nickels, and pennies aren't for babies. :)

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Use Littlest Pets or Polly Pockets or Squinkies for your manipulatives. Bet she'll like that.

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We did it with dimes and pennies and playing store. "This doll is 8 cents. You have one dime and two pennies. What are you gonna do?"

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15 - 7: first subtract to 10, and then subtract what's left. So first you do 15-5 (to get to 10) and then you still have 2 of your 7 left over. Subtract 2 from 10 to get 8. In order to do this kids have to know how to break numbers into their component parts.

It sounds more complicated typing it out than it really is.

This is how MEP teaches it too. I think it's really helpful to learn habits of composing and decomposing numbers like this.

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For 15 - 7 we do 15 - 5 - 2, making a note that the 5 and the 2 add up to the 7. That's worked really well for my non-mathy DD. :)

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