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


David Meego - Click for blog homepageAfter posting Trending: Only for genius ?? 3 – 3 x 6 + 2 = ?? yesterday, I had to post a follow up article with the correct answer.

Even with the hint about remembering the order of operations, people still got it wrong on my poll. If you haven’t done the poll yet, click on the link above and respond.

The question was

Maths_Question

The correct answer is as follows:

  1. Question: 3 – 3 x 6 + 2
  2. Multiplication first: 3 – 18 + 2
  3. Left to right: -15 + 2 (or Addition first: 3 – 16)
  4. Answer: -13

[Start Edit] I have been asked in the comments numerous times, why step three just calculates left to right? “Shouldn’t you do addition first?”. So I am adding this explanation section to the top of the article (it is similar to information in the linked related posts).

Order of operations (excluding the first two parts of Parenthesis/Brackets and Exponents/Orders/Indices) is [DM] or [MD] followed by [AS] or [SA]. So, Multiplication and Division can be done at the same time and then Addition and Subtraction can be done at the same time.

Try calculating the following examples from left to right:

Multiplication & Division: 4 x 3 / 2 = 6 and 4 / 2 x 3 = 6 …. The order is irrelevant.

Addition and Subtraction: 10 – 5 + 2 = 7 and 10 + 2 – 5 = 7 …. The order is irrelevant.

In fact, Division is just Multiplication of the inverse number. For example: Dividing by 2 is the same as multiplying by 1/2.

Also, Subtraction is just Addition of a negative number. For example: Subtracting 10 is the same as adding -10.

Maths is exact, that is one of the beautiful things about it. As long as you do it correctly you will always get the same answer.

For those of you who still insist on doing addition before subtraction:

  1. Question: 3 – 3 x 6 + 2
  2. Multiplication first: 3 – 18 + 2
  3. Addition next: 3 – 16
  4. Answer: -13 … the same answer as before.

Please note that -18 + 2 is not -20, it is -16. You cannot ignore the minus sign in front of the 18.

Also, 3 – 18 + 2 is not equal to 3 – (18 + 2) it is equal to 3 – (18 -2) because there is a minus sign in front of the brackets to the sign of the 2 must be changed when moving it inside the brackets.

If you don’t believe me, try solving 3 + 2 – 3 x 6 or 3 + -3 x 6 + 2 which are equivalents to the original equation.

All numbers (except for zero) are either positive or negative. Negative numbers have a minus sign in front of them and positive numbers have a plus sign in front of them. But, because we are lazy, we normally don’t write the plus sign unless it is between two numbers.

So 3 – 3 x 6 + 2 becomes +3 -3 x +6 +2 = +3 -18 +2. Now you can use a Number Line to finish the calculation.

using-a-number-line

The Answer still is and will always be -13.

[End Edit]

So what do you get if you type it into a calculator (physical or app)?

That depends on if the calculator is simple and calculates as you type, or if it is smart and waits for the entire equation to be entered and the equals button pressed.

On my Windows 10 Mobile phone, the calculator app can behave in both ways.

In Scientific mode, it allows the entire equation to be entered and so correctly calculates the answer as -13 using the “Order of Operations” rules (same as above).

In Standard mode, it calculates as you type and so incorrectly gives the answer as 2. The way to think of it is that it inherently presses equals each time you press an operation button. This is just left to right calculation and ignores the “Order of Operations” rules.

  1. 3 = 3
  2. 3 – 3 = 0
  3. 0 x 6 = 0
  4. 0 + 2 = 2

If your calculator behaves in this simple way, you would need to apply the rules yourself. Here are the buttons to press:

  • 3 x 6 = MS
  • 3 – MR + 2 =

If your calculator has brackets or parentheses buttons you can use them instead of the Memory buttons to achieve the same effect.

  • 3 – ( 3 x 6 ) + 2

Looking at the poll results from the previous article, there are a number of incorrect answers being selected, so I thought I would try to identify the actual equations that would give those incorrect answers (changes highlighted in red):

  • -17 = 3 – ( 3 x 6 + 2 )
  • -17 = 3 – 3 x 6 2
  • 0 = ( 3 – 3 ) x ( 6 + 2 )
  • 2 = ( 3 – 3 ) x 6 + 2
  • 8 = 3 – 3 + 6 + 2
  • 13 =  3 + 3 x 6 2
  • 17 = + 3 x 6 + 2

Hope this all makes sense. Here endeth the lesson. 🙂

Have a look at the other posts in the series:

Enjoy

David

03-Dec-2016: Added additional explanations at the top of the article.

05-Dec-2016: Added more additional information on number lines.

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

238 thoughts on “Answer: Only for genius ?? 3 – 3 x 6 + 2 = ??

  1. You clearly need to go back to grade school pemdas buddy 3×6 first then minus your answer which would be 18 by 3 which wiki give yo 15 then add 2 for a final answer of 17

    Like

    • Victor, I am sorry but you are WRONG.

      3 – 3 x 6 + 2 = 3 – 18 + 2 = -15 + 2 = -13
      or
      3 – 3 x 6 + 2 = 3 – 18 + 2 = 3 – 16 = -13

      You cannot ignore the minus sign in front of the second 3.

      Try summing this… in vertical format:

      3
      -18
      2 +

      ?

      David

      Like

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