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# Basic Math/does order matter when subtracting with regrouping

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QUESTION: Hi, I am homeschooling my children and have a question about subtracting and regrouping. We are working on regrouping and I think my daughter is getting it. However, my question is... After the regrouping is complete, does it matter (or will it matter for future math equations) what order she subtracts in? For example if the problem is
91
-39
----
and she regroups to get 80+11 Does it matter if she then subtracts from the 80-30 first? I know that as math gets more complicated order does matter and will affect the outcome of the problem but for this basic math, does it matter? It just seems like she is working it out that way first.
thanks for your time!
Chrissie

ANSWER: Hello Chrissie,

The order does not matter (as long as the signs are kept in the proper places).
In fact, it may be wise to demonstrate to her that the order does not matter, as it
illustrates the important property of commutivity.

Good luck!

Abe

---------- FOLLOW-UP ----------

QUESTION: Thanks Abe for your response.
I just wanted to double check that it doesn't matter in subtraction after re-grouping. I know commutative property doesn't matter in addition or multiplication. I would think it wouldn't matter if she subtracted the hundreds place before the tens and ones. But just wanted to double check. :)

Thanks again

Answer
Hello again Chrissie,

Of course, subtraction is not commutative, but if you treat subtraction as
addition of a negative, then it is...that is why I indicated
"... (as long as the signs are kept in the proper places)."

For example: 50 - 12 = 50 + (-12) = (-12) + 50 = -12 + 50

See?

Abe
Questioner's Rating
 Rating(1-10) Knowledgeability = 10 Clarity of Response = 10 Politeness = 10 Comment Thanks Abe! You have been very helpful. I think she will be happy to know this information. You were very quick in responding!!

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#### Abe Mantell

##### Expertise

Hello, I am a college professor of mathematics and regularly teach all levels from elementary mathematics through differential equations, and would be happy to assist anyone with such questions!

##### Experience

Over 15 years teaching at the college level.

Organizations belong to
NCTM, NYSMATYC, AMATYC, MAA, NYSUT, AFT.

Education/Credentials
B.S. in Mathematics from Rensselaer Polytechnic Institute
M.S. (and A.B.D.) in Applied Mathematics from SUNY @ Stony Brook

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