Quick Maths Question

Andy100

Andy100

Andy100

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I won't pretend to be great at maths but I thought I could answer this correctly...

Solve: 3-1+5


I thought the answer was 7.

An excel forumla and my scientific calculator would appear to agree.

An article I was just reading in a magazine says the answer is -3.

They are justifying the answer by saying BODMAS means you add the 1 & 5 before subtracting that total from 3.


Do all of our maths whizzes agree?
 
Add and subtract just go left to right, so I would say the answer is 7.

Now, if it was 3-1*5 then BODMAS would come into account and you would do the 1*5 first and subtract that from the 3, giving -2 but for add and subtract without any brackets it always goes left to right.
 
I'd always go left to right if there are no brackets and I'm fairly sure that's the way all maths software works. I just checked Matlab and google, both say 7.
 
Thank God I'm not going insane. :)

It was a computer magazine here in Oz.

A know it all was saying that the validation equations on some websites were accepting the incorrect answer (7).

I was 99.999% sure he was wrong but I thought I'd better Google and ask on here before I wrote in a scathing reply.

If only he had done the same...
 
That's what brackets are for (no not Angus-Higgins), to divvy up bits of an equation. Without any brackets I would just keep working it out from a running total so get 7.
 
I can see what he's getting at as if you follow BODMAS to the letter it would be 3-1+5
= 3-6
= -3

However BODMAS doesn't work like that. It doesn't actually mean all addition comes before subtraction. It means that addition and subtraction happen after brackets, after division and after multiplication, from left to right, so the answer is (3-1)+5
= 3-1 (+5)
= 2 + 5
= 7

You wouldn't start reading right to left (unless you were Japanese) so why interpret a maths question that way?
 
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Addition and subtraction have the same precedence, just like division and multiplication do, they've obviously misunderstood BODMAS. BOMDSA would be a valid acronym too, it's just you can't pronounce that as easily :p
 
BMDAS

Bless my dear aunt sally

brackets / multiplcation - Division / addition - subtraction

The foll has misunderstood, and is assuming you take addition before subtraction, well in that case, you do
-1 is a number, it isn't 1+5, it's minus 1 plus 5, leaving 4, adding 3 gives 7.


-edit for retardedness
 
Last edited:
An exception to the BIDMAS rule is presented when only additions and subtractions remain. From this point you would work left to right.

It looks like the article has failed to include this exception.
 
Saetia said:
I can see what he's getting at as if you follow BODMAS to the letter it would be 3-1+5
= 3-6
= -3
Click to expand...

That argument still doesn't work.
3 + [(-1) + 5]
= 3 + 4
= 7.

The fact of the matter is the sum shows the property of associativity and commutativity and so changing the order of operation will not change the final answer.
 
killari said:
That argument still doesn't work.
3 + [(-1) + 5]
= 3 + 4
= 7.

The fact of the matter is the sum shows the property of associativity and commutativity and so changing the order of operation will not change the final answer.
Click to expand...
The author of the article incorrectly assumed the - in front of the 1 as a minus operator instead of a negative prefix:

3 - (1 + 5)

Which is wrong, but it is understandable how they reached their answer knowing they applied BODMAS.
 
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