Missing numbers in addition and subtraction

[Apryl H]

Help :)

We are going over missing numbers in addition and subtraction (43-N= 12 , N + 13=35, etc.)

One of my dd's (4th grade) is having trouble grasping the concept of this. She's always been great at math, so I'm not sure where the trouble is. She will, using the above problem for an example, do this:

43-N = 12
12-43 = N

Then of course she runs into trouble trying to subract 43 from 12

I cannot get her to understand the order in which it needs to go:

43-12 = N

Any suggestions? She does ok with the addition side, but is struggling with the subtraction. The way they have it explained in Saxon is confusing to her for some reason. I'm thinking about giving her the full out algebraic formula for it and see if that helps, but I'm afraid I'll just confuse her further.

43 - N = 12
43 +12 = 12-12 + N
55=N

:confused:

[Snickerdoodle]

In Singapore Math they talk about the whole vs the parts. For the problem

43- X = 12

The child has to decide which number is the whole and which are the parts and write them as number bonds.

43
/ \
12 X

Then at the early stages if my son cannot figure out how to write a number sentence that will help him to solve X, I have him write out the entire number sentence family for those number bonds:

12 + X = 43 Part + Part = whole
X + 12 = 43
43 - X = 12 Whole - part = part
43 - 12 = X

and then from there he can decide which number sentence will help him solve for X. But I also want him to realize that addition and subtraction are just playing around with numbers that represent a whole and numbers that represent parts of the whole.

HTH

[Apryl H]

Thanks! I'll try to add that to the explanation and see if that helps.

[EKS]

Both of my children had difficulty with this. I got out the manipulatives to show how everything relates to the whole. Those linking cubes in groups of 10 work well for this because break apart easily. An abacus would work too.

So for your problem: 43 - N = 12

Start with 43 (4 strips of 10 and 3 more, you don't want to have to count things individually).

Say something like, "I start with 43 and I take something away and I end up with 12."

At this point you could have her take the something away to end up with 12. If she doesn't understand, then do it for her.

Then have her find out that the something was 31. Then start again with the cubes in two groups, 31 and 12. Tell her if I just know these two numbers 43 and 12, how can I find out what I take away from 43 to get 12. At this point it might become apparent to her that you could take 12 away from 43 to get the answer. If not, tell her and then repeat until she internalizes it.

This approach also works for missing addends.

HTH!

[Mallory]

What works for my boys is food.

This side is how many cookies you have 12
How do we get your brothers side the same? 43 - N

Yes we need to get 43 down to 12, okay if I take away 31, then you both have the same number.