Conclusion: Only for genius ?? 3 – 3 x 6 + 2 = ??


David Meego - Click for blog homepageLast week, I posted a couple of articles with a maths problem that has been trending on the Book of Face. If you have not read them and answered the poll in the first article, please do that now:

The reason for this third article is that the poll shows that less than 50% of the respondents answered correctly.

Many of those who understand the concept of Order of Operations (do the multiplication or division first) failed to apply it properly and of those who did apply it correctly, then failed to perform the addition or subtraction parts correctly.

So here is the question again:

Maths_Question

And here is the answer broken down into steps:

  • 3 – 3 x 6 + 2

Apply the Order of Operations. The mnemonic I remember is BIMDAS (Brackets, Indices, Multiplication/Division, Addition/Subtraction), which for this simple equation just tells us to do the multiplication first:

  • 3 – 3 x 6 + 2
  • 3 – 18 + 2

Now we can just perform the addition and subtraction as I read the line (from left to right):

  • 3 – 18 + 2
  • -15 + 2

Remembering number lines, with negative numbers on the left of the zero and positive numbers on the right. If I have a negative number and add a positive number to it, it will make the result less negative.

  • -15 + 2
  • -13

So the correct answer is -13.


The most popular incorrect answer was -17. This is the answer you will get if you correctly apply Order of Operations, but then incorrectly add 2 to -15. Below is an example to help you with this:

If I lend you $15, so you owe me $15. You then pay me back $2, you now owe me $13. Not $17.

[Edit] Also, I have seen 3 – 18 + 2 incorrectly written as 3 -(18 + 2) = 3 – 20 = -17, however there is a minus sign in front of the brackets and so just adding the brackets has flipped the sign on the number 2.

To be correct: 3 – 18 + 2 = 3 – (18 – 2) or 3 + (-18 + 2) = 3 – 16 = -13.

You can think of the sum as 3 + -18 + 2 if that helps.

The next most popular incorrect answer was 2. This is the answer you will get if you ignore Order of Operations and just calculate each part of the equation from left to right.

  • (3 – 3) x 6 + 2 = 0 x 6 + 2 = 0 + 2  = 2

The last of the most popular incorrect answers I will discuss is 0. This is the answer you will get if you incorrectly apply Order of Operations and perform the addition or subtraction before the multiplication or division.

  • (3 – 3) x (6 + 2) = 0 x 8 = 0

So why is this number line stuff so important… other than meaning that you can solve simple equations?

The reason is that it is the basis of accounting and that is what most of you reading my blog will be involved with in some way (Considering that I have based my career around ERP systems and specifically Microsoft Dynamics GP).

In my next couple of articles, I will discuss how I learnt accounting principles and the maths behind them (including number lines). I will also give you another maths challenge and see if more of you can get the correct answer.

Have a look at the other posts in the series:

Enjoy

David

This article was originally posted on http://www.winthropdc.com/blog.

17-Feb-2016: Added other explanation of why people are getting -17.

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26 thoughts on “Conclusion: Only for genius ?? 3 – 3 x 6 + 2 = ??

  1. Do I get points for a wrong answer if I didn’t learn this in school? Lol
    I don’t get how it can be a negative number though. How did the 18 turn into a negative?

    Like

    • Hi Tina

      Did you not learn to multiply first at school? They should normally spend some time discussing order of operations. PEMDAS or BODMAS or something similar.

      Now 18 did not turn negative, it was always negative because the 3 was negative.

      Subtracting a number is the same as adding a negative number. So if you think of the question as 3 + -3 x 6 + 2 = ?. Then -3 x 6 = -18. So it becomes 3 + -18 + 2 = ?

      Then use a number line (negative on left, zero at the middle, positive on right). …, -4, -3, -2, -1, 0, 1, 2, 3, 4 …. etc. Subtract positives or adding negatives moves left on the line. Adding positives or subtracting negatives moves right on the line.

      Starting at 3 add -18 (or subtract 18) = -15. Then add 2 = -13.

      David

      PS: Sorry no points for wrong answers. Maybe half points if you show your workings. 🙂

      Like

    • Hi Peter

      You just solved 3 – 3 x 6 – 2 correctly. Now can you solve 3 – 3 x 6 + 2?

      Addition and Subtraction can be performed at the same time and you will always get the same result if you do it correctly. The order is irrelevant as long as it is after Multiplication and Division (which can also be performed together).

      So you have 3 – 18 + 2

      Subtraction first (Left to Right): 3 – 18 = -15, so -15 + 2 = -13

      Addition first (though it is not required): -18 + 2 = -16, so 3 – 16 = -13

      Your issue is that the sum is NOT 18 + 2 = 20, but is -18 + 2 = -16

      If you don’t believe me, solve 3 + -3 x 6 + 2… which IS the same as the original equation.

      David

      Like

  2. In public school I was taught BEDMAS, meaning addition before subtraction. If there were brackets around a value with addition being the verb, that would change the + to a -, however, there were no brackets around the remaining “3-18+2”, so why would we not add before subtract in this case?

    Like

    • Hi Shannon

      The order you perform the addition and subtraction in is irrelevant, the answer will be the same. Subtraction is just adding a negative number.

      Just add 3, -18 & 2 together… The answer is -13.

      The mistake many are making is to ignore the fact that the middle number is -18 not 18.

      David

      Like

      • If this is the case, should there not be brackets around the (-18+2) portion of the equation, leaving 3-(-18+2)?? That would give 3+(-16) ? Or would that change the answer to -19 because the plus & minus would cause a minus??

        Like

      • Then shouldn’t there be brackets in order to get -13? Otherwise, would we not add before subtract as in the case of BEDMAS?

        Like

      • Hi Shannon

        6 – 3 is the same as -3 + 6 equals 3.

        The order of subtract and addition is irrelevant. You will always get the same answer regardless of which you do first.

        David

        Like

      • Hi Shannon

        There is no need for brackets to control addition and subtraction as the order they are performed does not affect the result.

        It is actually BE(DM)(AS). As division and multiplication can be performed together and addition and subtraction can be performed together.

        If that does not answer your question, please state it again.

        David

        Like

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