# Simple math question

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I need to explain to my son why

A -B + C = (A-B) + C but not A - (B + C)

He is only 8 and only in Singapore 4A. So I need to explain it fairly simply. The reason it came up is that he did a problem wrong and I realized that the reason why is he did it the second way (added B and C together and then subtracted from A).

I can just give him the rule that you have to do all subtraction and addition left to right. But I'm trying to understand better myself WHY so that I can explain it better to him. This is the kind of thing that was left out of my own math education.

I figured I could also get a bunch of manipulatives out and we could play with them to show him visually.

Still, I thought maybe there would be some math guru on here that could explain the why behind it to me. :)

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This is order of operations and I actually asked much the same question here a month or so ago and got a good set of resources and a nice answer. Here's that thread if you're curious.

I felt like I could follow order of operations but I didn't get the why behind it (and was therefore always forgetting it). While I got a nice answer there from Dana that helped me get it - I also learned that to some extent, it's just the convention - math people have just decided that's the order.

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Oh, thank you! You know I should have thought to do a search here. I did search for order of operations on google and found a few sites but they were either of the here's the mnemonic variety or really really complicated math that was just not going to help me explain it to my son.

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I need to explain to my son why

A -B + C = (A-B) + C but not A - (B + C)

He is only 8 and only in Singapore 4A. So I need to explain it fairly simply. The reason it came up is that he did a problem wrong and I realized that the reason why is he did it the second way (added B and C together and then subtracted from A).

:)

A - (B + C):

You are saying with parenthesis that you want to add B and C first, then subtract that sum from A. If you do that, you have to subtract everything inside the parenthesis, yes?

When grouping (A-B) + C, you are only subtracting B from A, because again, you do what is inside the parenthsis first.

In the original problem, A - B + C, the only thing being subtracted is B. C is being added as you work from left to right, one item at a time. You can do this with manipulatives, or on a number line.

Hope that helps :)

Jen

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