Simple math?

This is one of those crazy things put up on Facebook.

From what i can tell most people are getting this wrong.

But you follow the rules for simple math --

Rule 1:
  First perform any calculations inside parentheses.
Rule 2:   Next perform all multiplications and divisions, working from left to right.
Rule 3:   Lastly, perform all additions and subtractions, working from left to right.

So it is       3-18+2= -13 
And yet people think I am wrong ??   I simplified it even more for them ---- 3+2 =5     5-18= -13 

Why are so many getting this wrong?  Or maybe I am wrong---- time to go back to grade school I guess.

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rickharris1 year ago

Like most of these things it is written to be ambiguous.

To makes it a valid problem brackets would need to be inserted to show mathematical priority.

Otherwise I agree with you the answer is best fit as -13

You can't work this problem left to right which is how they get 2

Vyger (author)  rickharris1 year ago

That was my first reaction, that it was an incomplete equation. Like a sentence with out punctuation. But if the rules are followed it does have a solution. However if it was presented by a student as an answer to a something like a word problem I would give it a C or D for a grade because it is not really complete.

g-one Vyger1 year ago


There is no missing punctuation. Please see the link in my first reply. Call it "order of operations", call it "operator precedence", or call it "BODMAS". It's a clearly defined scope of interpretation for this equation.

As you say it is like miss punctuation.

I always recall this given as a punctuation exercise:

In came Caesar on his head, his helmet in his eye. A fiery look upon his feet. His Boots.

Ill ley you work out what the punctuation should be.

3-18+2= -13


3x6=18 3-18=-15 -15+2=-13

Im pretty sure your right what most people have done is forgotten that it has to be left to right and gone:

3x6=18 18+2=20 20-3=17

petercd1 year ago

Answer is 17 according to the BODMAS rule we were taught in school.

brackets of division, multiplication addition subtraction.

I think you have missed that the 3 x 6 is -3 x 6 giving -18

-18+2 is -16

3-16 = -13

Therein lies the ambiguity, however you cant reuse the same operand twice.

So technically its 3- 3x6 +2

and thus 3- 18 +2 = -17

According to BODMAS you should do the addition first.


+3-16= -13

There may be an argument to say +3-18 is an addition but it's a bit weak.

Why oh why does nobody seem to read my first reply at the end of this thread :-(

"It is helpful to treat division as multiplication by the reciprocal (multiplicative inverse) and subtraction as addition of the opposite (additive inverse)." [Wikipedia]

Multiplication over Division is wrong because Division is Multiplication.

Addition over Subtraction is wrong because Subtraction is Addition.

Thus, BODMAS should rather read B,O,DM,AS.

g-one petercd1 year ago

Subtraction and Addition have the same level and are executed left to right.

Why? Because Subtraction is technically Addition of a negative number.

3 - 3 x 6 + 2 == 3 - (3 x 6) + 2 == 3 - 18 + 2 == 3 + (-18) + 2 == -13

No ambiguity present.

g-one petercd1 year ago

42 is the alphanumerical sum of, "to be".

T=20 O=15 B=2 E=5

20 + 15 + 2 + 5 = 42

If you pay attention to the story line of the Hitchhiker's Guide to the Galaxy, it is just a remix of the Zen story of a mans search for the meaning of life.

What is the meaning of life?

To be.

Maybe because you did the work in a manner they could not follow.



But if I were you don't go back to grade school or you might think you are wrong.

Vyger (author)  Josehf Murchison1 year ago

I don't think I would fit in the desks anymore. And from what I understand they no longer have nap time and that is something I couldn't do without.

What would be really fun is to take in my slide rule and show how you can get answers with out needing buttons.

Take your slide rule to a college and watch all the PHDs be amazed at you getting all the answers without buttons.

These "simple" math things, at least here in AU, are spoiled by teachers and schools IMHO.
When I was at school we learned by writing with a pen, pencil and even ink.
It is a natural thing to for the brain to keep things active you wrote down but now schools work against this by insisting on the use of calculators for every tiny calaculation.
In grade my 5 my niece had to get her first calculator - a model the school insisted on.
This damn thing had no clue about math rules and your calculation would result in 2 as the incorrect answer.
Her new calculator for grade 7 knows math rules but only if you help it so you would know the rules to apply them like this: 3-(3+6)+2= ?
To make things worse up until grade 7 teachers insisted that there is no such thing as negative numbers so the kids won't be too confused if they make a mistake.
I tried my best with my niece but it seems the teacher is always right, the uncle never ROFL
The teacher tried to explain the negative numbers with a center ruler.
All positive numbers go to the right, the negative to the left.
Considering the kids always knew there are no negative number he has a hard time getting through.

So for all struggling teachers out there a simple explanation for negative numbers:
If 5 people are in a clsed room and 8 people come out, then 3 people have to go back in so the room is empty.

Vyger (author)  Downunder35m1 year ago

I just ran it on a calculator app on my I
pad and it got it correctly. Maybe those using smart phones are
getting the wrong answer.

I have always done the simple math in my
head. I really don't understand the need to always use a calculator.
When at the grocery store I run a rough rounded off estimate of the
contents of the cart and usually come up pretty close. My kids usually
can't do it and are often surprised by the total. Maybe the schools
should do a special course on figuring out shopping cart totals. Most
likely it would be lobbied against by the large chain stores.

The win 10 calculator thinks the answer is 2 so that explains a lot!

I got -13 too

and actually rule one is exponents

Yeah. The answer you get depends on the order in which the operations are performed, and there are rules for this, as you have mentioned.

So getting the right answer, for a problem like this, depends on two skills: (1) the ability to do the underlying operations correctly, and (2) the ability to do those operations in the correct order.

By the way, I fed this expression, "3-3*6+2" into the command line of a copy GNU Octave (, I've got running here, and it said:

>>> 3-3*6+2
ans = -13

Also, I should provide a link to an online version of octave,

for anyone reading this who wants to try out that expression, or test Octave in other ways, without the hassle of actually installing Octave on his or her local computer.

So, Octave says you're doing the math right. Although I doubt that's going to settle the debate with people who think they can do the operations in whatever order they feel like, and still get the right answer.

There is old internet proverb that says, "Arguing with people on the internet is like running a race in the Special Olympics. Even if you win, you're still a retard"

g-one1 year ago


If your question is if you're right or wrong then the answer is you're right. The result is -13. I can't answer your question why people think it's wrong. It's simple math!

For those who don't believe it