MM2 Subtraction

[Ruby Rose]

We have just started the subtraction portion and it teaches it my breaking down the problem to tens.

 

14-8=_

14-4=10

10-4=6

 

So 14-8=6 :001_huh:

 

Seems overly complicated to me. I'm willing to stick it out if it is the best way to teach the concept and is built upon as we progress.

 

What says the Hive?

[Nart]

I am just starting to cover subtraction of numbers over 10 in Singapore Math with my son. It seems like in SM that

14-8 would be to decompose the 14 (into 10+4) not the 8 (into 4+4):

(10+4) -8 =

10-8 =2

2+4 =6

[Rasa]

In my experience, MM presents many different ways to solve one problem. My son usually has a preferred method, but I ask him to use the recommended approach in at least several problems. Occasionally, the new method becomes his favorite, but if it does not, we move on, as long as he CAN solve the problems correctly and efficiently using his preferred way. (For full disclosure, we use MM Blue, not Light Blue, series.)

[Farrar]

I make sure my ds can do the way they're presenting, but if he doesn't want to rely on that method, that's okay. I do think learning all these different ways to do the problem (decompose the minuend, add to both the minuend and the subtrahend, decompose the subtrahend, etc.) are all valuable at different times and therefore it's best if they're fluent in all the ways. However, sometimes MM's extra steps seem to get in the way for some kids.

[SorrelZG]

When you do it in your head it's not complicated. Ultimately it only takes a split second because you're mentally seeing 8 as 4 and 4 straddling the 10 (for example). It only becomes complicated when you have to put it into words and write it out step by step on paper (which you do to explain it).

 

Out of pure curiosity, because I like to know how other people think, how do you (personally) work through such problems?

[Dinsfamily]

I am just starting to cover subtraction of numbers over 10 in Singapore Math with my son. It seems like in SM that

14-8 would be to decompose the 14 (into 10+4) not the 8 (into 4+4):

(10+4) -8 =

10-8 =2

2+4 =6

 

Although we use Singapore instead of MM, ds8 naturally does it the way it is presented in the OP. I guess it makes more sense in his head. I also developed that method even though I was never taught to do mental math. The SM method seems to work better for ds6, but I'm teaching him both.

 

To the OP, it becomes quite natural and not complicated with practice. It is also applicable to do mental math with larger numbers. I wish I'd been taught that way from the beginning. Ds8 is much faster with mental math than I am.

[crazyforlatin]

When I see what MM and SM teach kids, I think a bunch of us at the parochial school were deprived. DD knows the different methods of subtraction and uses the method that best suits the problem. I think the goal here is to be mentally flexible, and to get there, you can't just have one way of looking at a problem.

 

As the PPs have mentioned, OP's way seems complicated, but in practice, it's the fastest way, I think, when you are mentally stuck.

[Ruby Rose]

Thanks! I'll stick with it then. My old brain is just :confused:.

 

To the question of how do I work it out in my head.

 

14-8=_

8 needs 2 so 4+2=6

 

Or

 

The same way as MM, but when teaching it it seems way more complicated than when your brain just automatically calculates it.

 

I'm all for a different method. My math skills were/are weak because I slipped through the cracks and was never properly taught. I want my kids to be STRONG and CONFIDENT in math!

[Farrar]

 

The same way as MM, but when teaching it it seems way more complicated than when your brain just automatically calculates it.

 

 

That's exactly why MM teaches it though - because it's not automatic at first. :001_smile: