The problem with PEMDAS

This problem has really stirred a lot of interest and created a buzz on the internet. I can see why, it’s an easy one to miss. And yet, PEMDAS is such an easy thing to remember, the mnemonic devices offered make for a strong memory. So people passionately defend their answers.

6 ÷ 2(2 + 1)

I am going to tell you the answer in just a moment, but before I do, please listen to why I think this is a worthy problem to explore.

There are two fundamental misconceptions with math that make math into a monster for so many people, and this problem touches on both. In a sense, neither has anything to do with the order of operations specifically.

The first issue is understanding that spatial arrangements in math mean something. The way we write the numbers and symbols has a meaning, very specific at that. In this video by Mind Your Decisions, https://youtu.be/URcUvFIUIhQ, he shares where there was a moment in time when we used different conventions to write math.

And while math may or may not be a human invention, the symbols and arrangements and their meanings certainly are. Just like the letter A is only a letter and with a specific sound because we all agree. Just like a red light means stop, a green light means go and a yellow light means HURRY HURRY HURRY!

The second, and more over-arching issue here, is the misconception that addition and subtraction are different. They are fundamentally the same thing. Subtraction is really addition of opposite numbers. Perhaps to shore this misconception negative numbers should be introduced instead of subtraction.

Now you might argue and say, Wait, addition has properties that subtraction lacks, like the commutative property.

You’re correct, 5 + 3 = 3 + 5, while 5 – 3 does not equal 3 – 5. However, 5 – 3 is really five plus the opposite of three, like written below.

5 + - 3

And that is the same as this expression below.

-3 + 5

So the AS at the end of PEMDAS is really just A, or S, whichever leads to the better nursey rhyme type device to improve recall.

Since we believe that addition and subtraction are different, we also come away with the belief that multiplication and division are different. Sorry, they’re not. Division is multiplication of the reciprocal. Remember that whole phrase from your school days? (How was that for a mnemonic device?)

And while division does not have the commutative property, that again is a consequence of the way we write math. If we only wrote division as multiplication of the reciprocal, we would see that multiplication and division are in fact the same.

So, back to the problem. The most common wrong answer is 1. The correct answer is 9. Here’s a great video on the order of operations, super catchy and articulates the importance of left to right as written for multiplication and addition.

Last thing: Now, in creative writing the intent of the author must be considered, should it also be considered here?

Let me know what about this you like, dislike or disagree with. Let me know what is helpful. I really want to promote success through making math transparent. It’s my mission. You can help support my mission by just sharing and liking this. Subscribe to my blog if you’re a teacher as I will be populating it with lots of teacher advice, not all math related.

Thank you again for reading.

If 6÷2 (1+2)=9 what is the answer to this? 6÷4 (1+2)?

Four and a half. I really think the easiest way to handle these is by rewriting the division as multiplication of reciprocal. Then you end up with the product of 6 and 3/4.