# How to Solve this Basic Math Problem

## Recommended Posts

How do I teach my 8 year old how to do these type of problems.

____ + 9 = 15

____ - 9 = 20; and

9 - ____ + 89.

I understand how to do the first two and feel comfortable explaining to him to just do the reverse operation, but I do not know how to explain how to do the third one (especially in a way that an 8 year old will understand).

Your advise is greatly appreciated. He is really struggling with these, especially when use use larger 2 and 3 digit numbers.

##### Share on other sites

For the first two, I would give the addition 6+9=15 show it to him, cover up the 6 with a piece of paper. Then ask him how he would know it was a 6 if he didn't see it first.

I have also used number cards and turned them over to "guess" which number was hiding. It makes it more of a game than a "problem" - same concept.

Cuisenaire rods are great for seeing the relationships between addition and subtraction.

##### Share on other sites

You need to emphasize that subtraction and addition show a relationship between a whole and its parts. You can use a model to help the child discover which are known and which are unknown. The conceptual understanding needs to be there; that the whole amount is a sum of its parts, and parts added together equal a whole amount. If you know your whole amount and one of its parts, you can subtract to find the other part.

Singarpore uses a "model method" where you draw a bar and label the total length (with a known number or a ?) and then divide it into sections (of known length or ? length).

So for the first question, your bar would have a bar with a line drawn a bit less than halfway through it, and in the first section you would write "?" the other section you would write "9" and along the total length you would write "15." This helps the child see that the amount "15" is composed of 9 and __ and they should be able to see that they subtract to get 6.

For the second problem, the total length is unknown ("?") and it can be broken into two parts: 9 and 20. The child needs to understand that if you take a whole amount and subtract one part, your answer is the amount of the other part. If you add both parts back together, you get your total amount again.

For the third problem (assuming you meant to write 9+ ____ =89) yo would draw a bar with the two parts labeled 9 and ? and then the total amount is 89.

Out of curiosity, which math curriculum are you using? It doesn't sound like your child has learned the "why" behind the math.

##### Share on other sites

How do I teach my 8 year old how to do these type of problems.

____ + 9 = 15

____ - 9 = 20; and

9 - ____ + 89.

I understand how to do the first two and feel comfortable explaining to him to just do the reverse operation, but I do not know how to explain how to do the third one (especially in a way that an 8 year old will understand).

Your advise is greatly appreciated. He is really struggling with these, especially when use use larger 2 and 3 digit numbers.

I just want to show you a nine trick that I figured out on my own and then my older son figured it out so, I only taught my younger son. Subtracting 9 is the easiest number because it is close to 10.

In the problem ____+9= 15, if you add the 1 and the 5 in 15, you'll get 6. So the answer to that problem is 6. I discovered that when subtracting 9s, the answer is always one up from the number 9 is taken away from. For example:

17-9= (1+7=8) So, the answer is 8

16-9= (1+6=7) So, the answer is 7

This trick works everytime. If you have a problem lik 27-9= ?? Always say what is one more than 7, it's 8. So, the answer is 18.

Blessings,

Karen

http://www.homeschoolblogger.com/testimony

##### Share on other sites

I use Prof B and teach it much like Andrea. First I write lots and lots of addition and subtraction problems on the white board and show that all of them have a "big" number and 2 "smalls" that equal the big. I write the problems in all different orders and we see that order doesn't matter, we always have 2 smalls and a big.

Then I tell them that in missing number problems the only thing the addition or subtraction sign are good for is to tell them where to find the big number. In addition problems the big is by itself on one side of the equal sign, in subtraction, the big is immediately to the left of the minus sign.

If you are missing a big you add the two smalls you do know, if you are missing a small you subtract the small you do know from the big. Takes a bit of practice to internalize but all mine got it.

HTH

Cindy

##### Share on other sites

In the CD, he presents a game called the "Circle Game." To play the "Circle Game", draw one big circle at the top of a page, chalkboard or white board, with two smaller circles below it, one to the right, and one to the left. The overall shape of the three circles in relation to one another will be a triangle, if that makes sense.

O

o o

Then play as Cindy suggested, filling in two of the circles, and having your dc "guess" what should go in the other.

I've also done this using magnetic letters with = and - signs, by having my dc rearrange them on a board to make as many math facts with three numbers as they can (eg. 1+2=3, 2+1=3, 3-1=2, 3-2=1).

## Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

×   Pasted as rich text.   Paste as plain text instead

Only 75 emoji are allowed.