do a simple maths problem

Post by **deject** » June 18th, 2010, 3:26

Do this maths problem. It's really easy, yet a surprising number of people get it wrong.

Solve this: 1 - 3 x 4 + 7 = __

Post by **FatherJack** » June 18th, 2010, 3:40

I say -1. C++ would say -18 and a spreadsheet would say -4.

Post by **deject** » June 18th, 2010, 3:54

we're not talking about any sort of machine logic, just doing it the way you were taught to do arithmetic.

Baliame · Post by **Baliame** » June 18th, 2010, 6:47

Code: Select all

1 - 3 * 4 + 7 = (-3 * 4) + 1 + 7 = -12 + 1 + 7 = 8 - 12 = -4
(this is not, in fact, a code)

C++ would likewise say -4 and a spreadsheet would definitely say -4.
Anyone who says otherwise should debate it on /b/.

spoodie · Post by **spoodie** » June 18th, 2010, 8:16

-1 is my answer. It's been a long time since I learnt arithmetic but that seems the logical answer.

Baliame's answer seems to be the application of too much logic.

Post by **fabyak** » June 18th, 2010, 8:50

I'll go with -4

Multiplication and division take priority over subtraction and addition (with things in brackets taking highest priority)

/A-level maths

friznit · Post by **friznit** » June 18th, 2010, 9:17

I still remember BODMAS from Fucking Years Ago™. -4.

spoodie · Post by **spoodie** » June 18th, 2010, 9:18

fabyak wrote:Multiplication and division take priority over subtraction and addition

As it's been a while for me, why is this?

Post by **fabyak** » June 18th, 2010, 9:23

spoodie wrote: As it's been a while for me, why is this?

Because you touch yourself at night

friznit · Post by **friznit** » June 18th, 2010, 9:25

It's just convention, so everyone knows to use the same order of operations applied to each variable without having to spam brackets everywhere:

Brackets
Order
Division
Multiply
Add
Subtract

Ed.: Note that Multiply/Div take the same precedence though.

Also, what Fab said.

Post by **ProfHawking** » June 18th, 2010, 9:35

I wouldnt be 100% sure how to do it unless it were in context.
I see three methods:

((1-3)*4)+7 = -1
1-(3*4)+7 = -4 I would go with this one personally
(1-3)*(4+7) = -22

If in doubt, use bracketspam

shot2bits · Post by **shot2bits** » June 18th, 2010, 9:49

ProfHawking wrote:I wouldnt be 100% sure how to do it unless it were in context.
I see three methods:

((1-3)*4)+7 = -1
1-(3*4)+7 = -4 I would go with this one personally
(1-3)*(4+7) = -22

If in doubt, use bracketspam

this is pretty much what i did in my head and i was about to post before i saw this, i came out with -1 from doing it how i would normally do maths and how i expect most people do instinctually due to the presentation of the question, then after rereading the question i saw the way to come up with -22 and then again i looked at it and saw the way for -4. i couldnt see any other it could work

spoodie · Post by **spoodie** » June 18th, 2010, 9:49

I agree with Prof and I've made up a new thing for maths people to remember:

Explicit
Or
Shut
The
Fuck
Up

amblin · Post by **amblin** » June 18th, 2010, 10:16

Post by **Dog Pants** » June 18th, 2010, 10:53

friznit wrote:It's just convention, so everyone knows to use the same order of operations applied to each variable without having to spam brackets everywhere:

Brackets
Order
Division
Multiply
Add
Subtract

Ed.: Note that Multiply/Div take the same precedence though.

Also, what Fab said.

This.

So:
1-3x4+7=X
1-((3x4)+7)=X
1-(12+7)=X
1-19=X
-18=X

buzzmong · Post by **buzzmong** » June 18th, 2010, 11:18

Multiplication has a higher precedence than anything else in this statement. Addition and subtraction have the same precedence as each other.

Bearing in mind this is an in-order operation, not pre or post order, therefore when faced with items on the same level, you go left to right.

So, 3x4 = 12

1 - 12 = -11

-11 + 7 = -4

Baliame · Post by **Baliame** » June 18th, 2010, 11:31

Dog Pants wrote:This.

So:
1-3x4+7=X
1-((3x4)+7)=X
1-(12+7)=X
1-19=X
-18=X

I sincerely hope you're trolling. Where did the extra brackets come from?

Let's see. First of all, we're dealing with addition here. Multiplication takes precedence over addition and subtraction, therefore we're we have three operands, as subtraction can be considered as the addition of a negative number. Therefore, the equation can be written as:
(1) + (-3x4) + (7) = X
Note that the brackets do not make a difference in this case, they're only there for emphasis:
1 + -3x4 + 7 = X
Now, to remove all confusion, and to make it easier to understand, we will execute the multiplication.
1 - 12 + 7 = X
Knowing you all, this equation still has 3 equally probable outcomes but before those rusty cogwheels start cranking again let's do the additions (and subtraction).
X = -4

If you're wondering why I'm bitter about this, I saw the exact same conversation go down on 4chan, except there people were actually trolling.

Post by **Dog Pants** » June 18th, 2010, 11:42

I just wanted to be different, but hey thanks for calling me retarded. Now go post all that on 4chan.

Baliame · Post by **Baliame** » June 18th, 2010, 11:43

Okay, that may have been a little harsh

But I find it a troubling question now that I think about it, I think I wasted 12 years of my life so far learning things I most likely won't even want to remember.

spoodie · Post by **spoodie** » June 18th, 2010, 11:47

I saw the exact same conversation go down on 4chan, except there people were actually trolling

That's because it's horrible site.

Personally I now understand the rules that are expected to be applied to this equation, but I still don't know why. It seems arbitrary. I don't particularly have much respect for this kind of pure maths logic.