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# Order is essential in the definition of multiplication because not all forms of multiplication are commutative, such…

Order is essential in the definition of multiplication because not all forms of multiplication are commutative, such as matrix multiplication. This is why it is taught as a separate property.

Originally shared by Science on Google+

Why Was 5 x 3 = 5 + 5 + 5 Marked Wrong?

It seems absurd at first glance: we all know that 5 x 3 is equal to 3 x 5, which is 15. But check out the formal definition of multiplication:

The multiplication of two whole numbers, when thinking of multiplication as repeated addition, is equivalent to adding as many copies of one of them (multiplicand, written second) as the value of the other one (multiplier, written first).

In other words, 5 x 3 = 3 + 3 + 3 + 3 + 3

Why should this matter? It matters because the term equal is not the same as equivalent. Although 5 x 3 is equal to 5 + 5 + 5 it is not equivalent to 3 + 3 + 3 + 3 + 3. Suppose you were buying chocolates for your sweethearts on Valentine’s Day. You would have 3 boxes of 5 chocolates each in one case, and 5 boxes of 3 chocolates in the other case. What you choose to buy depends on how many sweethearts you are trying to impress, right?

Perhaps more importantly, the difference is also a fundamental concept in computer science.

Notice that the second problem is marked incorrect as well. That’s because keeping rows and columns straight in matrix multiplication is important. As explained in the link below: “Order is essential in the definition of multiplication because not all forms of multiplication are commutative, such as matrix multiplication. This is why it is taught as a separate property.”

So, what do you think? Do you agree with the teacher or not?

https://en.wikipedia.org/wiki/Multiplication

https://medium.com/i-math/why-5-x-3-5-5-5-was-marked-wrong-b34607a5b74c

## Join the Conversation

1. No. Fuck the teacher. 5*3 is equal to 3*5. Your mathematical bs rules be damned

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2. I don’t agree because 5×3 means 5, 3 times. If it were 3×5 that would mean 3, 5 times.

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3. I think, at the primary level, this is clearly pedantic. What kid knows the difference between equal and equivalent? How many of them have even heard the word “matrix”?

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4. This also was not computer science. It is basic multiplication.

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5. Noah Hester the post does not state that problem 2 is matrix multiplication. It explains why rows and columns are not interchangeable, which is why the answer was marked wrong.

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6. C Ames the point that the poster is making is that the distinction between 5 x 3 and 3 x 5, i.e. that of non-equivalence, becomes important later in computer science. That’s why the concept is important early on. The second link has a longer explanation.

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7. At the primary school level, marking wrong for order is blithering idiocy. You’re only dealing with scalars at this point, and for scalars multiplication is commutative. In fact, that’s one of the ways that kids figure out what commutative is when doing scalar arithmetic: multiplication and addition are, subtraction and division aren’t.

There are also overloaded uses of the + operator where A+B B+A, but we don’t mark kids down for not knowing that until we’ve taught them the different operations in the first place.

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8. WTF!?!?!?!  Equivalence has absolutely nothing to do with either of those two questions.  The teacher really is an overly pedantic idiot.  Whomever wrote the attempt at justification above is way off base too.

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9. Sorry, but in software development we do optimizations like that all the time.  If you can refactor a an algorithm to use fewer CPU cyles and still come out with the same answer, by all means do it!  (Assuming it doesn’t impact program maintainability etc.)

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10. These concepts are taught in Grade 3 as described in Common Core standards:

Students develop an understanding of the meanings of multiplication and division of whole numbers through activities and problems involving equal-sized groups, arrays, and area models; multiplication is finding an unknown product, and division is finding an unknown factor in these situations. For equal-sized group situations, division can require finding the unknown number of groups or the unknown group size. Students use properties of operations to calculate products of whole numbers, using increasingly sophisticated strategies based on these properties to solve multiplication and division problems involving single-digit factors. By comparing a variety of solution strategies, students learn the relationship between multiplication and division.

Check it out: http://www.corestandards.org/Math/Content/3/introduction/

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11. Indeed the kid should get extra points or a gold star for possibly demonstrating the commutative nature of multiplication!

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Note also w.r.t. q#2 on the paper this from the page you refer to:  _”Students understand that rectangular arrays can be decomposed into identical rows or into identical columns”_

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13. Sorry, not reading what correctly, Greg A. Woods ?

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14. Why negate ? psychology studies again and again say that “reward” is a much better motivator. If I were the teacher, I’ll give those two questions +1 each, and if they write down both 5+5+5 and 3+3+3+3+3, I’ll give them +2 points.

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15. I think this is overly pedantic and is against any the associative property, giving further confusion down the line while learning mathematics. With such strict rules about what multiplication is, how do you go about teaching kids how to solve for x?

5x = 15

“You can’t just move numbers around!” I already hear this enough from people who don’t understand math very well. I can only imagine how terrible peoples understanding of math will be when you constantly give them rules and change them.

I also agree with the first comment on the medium link:

“However, (attempting to play devils advocate), I would say that the only downside to this method of teaching math is that it squashes ANY chance at creativity. Math is already a subject with little room for individual creativity, and this method eliminates any chance at that. It shows the children that unless they do the problem EXACTLY the way the teacher wants, they are wrong. What does this do to kids as they get older?How are they going to solve the worlds problems if they are limited to only solving problems the way their bosses see fit? As you mentioned, it does have benefits (for CS and matrices), but looking at the larger picture, how valuable is creativity in this day and age?”

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16. Nerds are crawling from their dundgeons to slaughter each other over 5*3 or 3*5!

Geez

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17. The boxes of chocolates analogy doesn’t really explain anything. 5 × 3 in such a context would solely pertain to the chocolates, not some new arbitrarily-introduced “sweethearts” value. Who’s to say that I didn’t build my own giant box and just got the boxes of chocolate to fill said box with to give to one sweetheart?﻿

Also, the commutative property is usually taught well before matrix arithmetic (at least in a sane education system); unless it is indeed the case that this is supposed to be some intro to matrices (which is not at all clear), the point is definitely pedantic anyway for what looks like elementary-school math homework.

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18. I have to mute nerd chatter

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19. Diimaa You seriously came to a post from the “Science on Google+” only to complain about “nerd chatter”? What did you expect? 🙂

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20. Let it to god. It’s just a math problem.

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21. That makes no sense and it’s perfectly ok, because when dealing with real numbers both multiplication and addition are commutative operations. Sure, in some algebras they aren’t, but when dealing with real numbers, they are.

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22. That is the most Bullshit response to why it was wrong.

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23. Bottom line not all teachers remain smarter than the pupil. Hopefully the parent is at the school seeing that the grade is changed. Lol looking at this the theme song for are you smarter than a 5th grader popped in my head.

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24. I’d also like to bring up that 5 and 3 are just a bunch of 1’s added to each other.

(1+1+1)+(1+1+1)+(1+1+1)+(1+1+1)+(1+1+1) = 15

(1+1+1+1+1)+(1+1+1+1+1)+(1+1+1+1+1) = 15

I’d like to bring up the argument that we are pretty much complaining about where to put parentheses.

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25. Probably a matter of circuital calculation is involved.

Reasons should be given to do ( or better prefer ) one given solution to another possible one.

Please give me reasons and I shall understand.

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26. interesting!

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27. A good teacher would have marked it correct but explained that while it’s correct it’s not the common notation.

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28. At first I read some of these comments and I could understand why you might interpret 3 x 5 as 5 three times, but then I realized that these are Arabic numbers! We read math and do it from right to left! Our primary schooling has given us the tools to get on with the learning but in fact or pronunciation is dyslexic.

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29. Like the example of sweethearts. I should say that I prefer 5 boxes of 3 candy each one than a set of 3 ones with 5 candy, because in the first case you have 5 sweethearts and not just 3 ones.

Is it BTW some kind of modal logic or just pure biased calculation?

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30. It’s common core math, no wonder why it’s so confusing.

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31. C Ames 5x means x, 5 times. Or you prefer 5, x times ?

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32. The teacher did the same thing on the next problem. Instead of four rows of six she made six rows of four. Different ways of teaching I guess.

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33. Noah Hester I do believe that this is 2nd GRADE work, so I don’t think that it matters, but look at it, the array the child did was 4 across and 6 down. The correct answer in the teachers mind was 6 across 4 down.

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34. No, it’s all the same answer.

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35. This whole exercise is why most of you will be poor all your lives.  For this way of teaching only teaches conformity and does not allow for independent thought.

Thus training an army of drones that cant think of new and inventive ways of doing things and instead tell everyone the same old song.  Do it our way or fail.

This is why our current Dewey/Rockfeller system is causing the failure of America.

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36. The results is the same no matter what, screw scientists when nature speak. ﻿

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37. Frank Copley Yes Sr. Good words, too bad some ignorant still not getting it.

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38. Michael Matzke​ “Common core” I believe was invented to screw more our young generation. They don’t want educate people.﻿

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39. Oliver Hamilton​ good point Olv.

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40. Congratulations. That kid is now going to major in Literature.

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41. This is just mathematics reading comprehension . Kids need to be in abstractive concepts to understand this slight difference: 5 bags of 3 apples 3+3+3+3+3 instead of 3 bags of 5 apples 5+5+5 …

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42. WHEN YOU GO TO REAL WORLD YOU NOTE THERE IS A DIFFERENCE.

I THINK IT IS THE POINT TO STRESS: Math is a tool. Is not a description of reality.

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43. Ccore? What a load of BS!

Math is fun, math is intuition, math is imagination… awe and wonder!

﻿not…ape the rules!

Kill them young …is what this will achieve.

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44. My brother smart algbra school…

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45. Five times three is 15 and also five plus five plus five is 15.

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46. The question is: Did teacher explain it before the test ?

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