Singapore 2B, Specific Lessons


Singapore 2B, Specific Lessons

Unread post by cbollin »

Singapore 2B subtraction question
Peter wrote:Hello. I was wondering if anyone can tell me if I am doing this correctly. Everything is pretty self explanatory but we came across one subtraction problem I was not too sure off. It is found in 2B workbook pg. 21 4. (a) 539-70. I was having him take 539 and breaking it down using that triangle symbol (^). We put 530 on one side and 9 on the other. We then mentally counted backwards and subtracted 530 from 70. We then subtracted mentally by counting backwards the remaining ones (9).
I hope someone can understand this! I am not good at explaining things very well. Thank goodness my son is a natural at math. He came up with the right answer. My concern is that it wiped him out mentally after subtracting the 530 from 70. He started to moan and groan. Any thoughts or another way to do this? He wanted to just write it out vertically. I just don't think I should allow him to do this because it defeats the whole purpose of the "mental math thing". Any advice would be greatly appreciated. Thank you!


Yes, he can write it out vertically. You don't have to do all problems mentally. And there is more than one mental process on it.

My oldest would have done mentally but it's exercise 9. so, my oldest daughter's method wouldn't have been taught.

In my middle daughter's workbook, she did not mentally do any in section 4 on p. 21. It turns out she didn't stack them though. She did it horizontally with regrouping -- just like you would with vertical, but she did it horizontally.

I definitely did not make her do that section mentally without help and modeling.

If you want to do it mentally, think of it as taking out groups of tens from the 39, not from 30. Jump back in segments.... do it in 40 then 30

modeled this out loud
first mentally take out 40
that gets you to 499
now you have 30 to go.

first take out 50
take 40 more

so don't take it from the tens' and then units

Then, if your child catches the pattern, continue to do them out loud together in that fashion. 353-70 becomes: 353 minus 50 = 303, go back 2 more rounds of 10's and you get 293, 283.

otherwise, it's ok to stack :)

I'm thinking I probably modeled some of this for my middle daughter by writing those on the dry erase board.

509 (giggling because I had to edit this number in, I got distractedwhile typing)
and then she'd get stuck and I'd just say
then prompt her for 489

so, if you want to practice it that way, that works too. Those lessons have been taught to this point.
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Joined: Sat Jul 23, 2011 12:46 pm

Re: Singapore 2B subtraction question

Unread post by Peter »

Thank you for those wonderful examples. You have been such an awesome help. Thank you for taking the time to respond. I appreciate it.

Julie - Staff
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Joined: Fri Nov 03, 2006 11:52 am

Subtraction trouble

Unread post by Julie - Staff »

Sallylourn wrote: Sat Aug 12, 2017 2:34 pm My daughter ended last year halfway through Singapore 2B, so we just picked up where we left off. She's starting 3rd grade and is pretty good at math, unless it has to do with subtraction with regrouping. I think she must have just convinced herself she's bad at that because she gets frustrated with it immediately whenever it comes up. She doesn't have her math facts memorized for subtraction yet, either. It is smooth sailing until we get to a subtraction-heavy section, and then math becomes a terrible chore with lots of tears. Once we pass that section, she sails through the next section and says it's so easy.

I don't know if it is right for me to keep going like that when she hasn't mastered subtraction with regrouping yet, but I've tried spending extra time on it, and it is always an emotionally charged and unpleasant time. Last year, I tried to just spend some time on math facts and skip doing the lessons for a while, but that is also something she dislikes and her attitude makes it difficult for me to stay positive.

I don't know if I should keep going or stop and spend extra time on subtraction. Also, I'm not sure how to go about it, and it seems like she doesn't want to hear it coming from me. I know she is ahead of where she needs to be, so we have time to take a break from Singapore for a while and concentrate on subtraction, but I have a feeling she wants a break from it (and I do, too!), and the next subject will be much easier for her and more relaxing for both of us, so I'm tempted to just push through to the next thing. I know this has to be dealt with at some point, though. I just don't know how. Your wisdom would be very much appreciated!
Hi Sarah,

It sounds like you're doing a good job of touching on the tough stuff and then backing away for some encouragement. In some cases, learning just involves exposure over and over until the brain settles the information into its place, and then it becomes easy.

When you see a double-digit subtraction problem coming up, you might start your textbook lesson back at the start of 2B, where the lesson shows the relationship between 7+5=12 and 12-7=5, etc. Those different groupings of 12 objects can be demonstrated using things like doll shoes or hotwheels cars, just reinforcing -- even by touching the objects -- to see that there are always 12 items total, however you combine the numbers. That idea has been there (connecting addition and subtraction) ever since level 1A, but now it becomes an essential understanding, whereas earlier it might have been not as necessary to understand :)

Some of the next 2B problems work with totals of 100. Pennies work well for this, or beads or beans - something you have at least 100 of. Help your daughter see 100 in a concrete way. Then take out a group and look at what's left - lining up the remainder in 10s. So for the problem 53 + ? = 100, try taking out 47 and lining up the remaining 53 into 10s, and see if she can figure out what's missing. If not, line up both the 53 and the 47 in groups of 10 and have her see the two groups equal 100 together. Things like that. Create a concrete place in her brain for these types of problems. And keep addition and subtraction connected together.

I'm not sure if she's trying to solve subtraction problems by stacking -- writing 100 at the top and 53 underneath. Stacked problems are taught in Singapore as a tool that students are free to use but clearly not the only way for kids to solve subtraction problems. In fact, if stacked problems are her weakest/wobbliest tool, then that is fine, since the preferred Singapore method is to solve horizontally.

And I just wanted to add that even my mathy youngest had days (and weeks) when he needed to just work on math facts or math games, setting aside a textbook topic for a while. He still has made it through all kinds of math in college, so all is not lost if the student needs a little break along the way, as long as you keep those math muscles flexing in some way or another.