BIDMAS question - what's the right answer

[Overstating]

7-5+6x13?

Thanks!

You dont avoid the minus until the end as the number is -5

It means 7 - (all the other sums done first). The 5 is part of the other sum. It's a 5, not a -5.

Anyway suggest OP checks with the teacher and comes back to resolve the argument.

Not going to get into a Mumsnet argument anyway but in my mind it makes sense and I don't think the teacher deserves a bashing.

She absolutely deserves a bashing. He job is to know this stuff so she can teach it correctly to children. If she can’t do it, she shouldn’t be in the classroom. I am a primary school teacher. This is fundamental basic maths.

It means 7 - (all the other sums done first). The 5 is part of the other sum. It's a 5, not a -5

Sorry, you're wrong here

I'm in agreement with those saying 80. The only way for it to be 7- (result of the other sums) would be if there were brackets in place.

D/M are of equal importance and done before A/S which are also of equal importance so can be done in any order

7-5+6x13?
If you rearranged this it would be

-5+7+6×13 as the + and - stays with the number

It’s like saying that “jaune” is French for green millefiori, because it reminds you of “jeune” and young and green are synonyms. It doesn’t matter how much sense the reasoning makes to you - you’ll be bugged trying to buy a cardi in Paris. And if your teacher had taught you that they’d need a bollocking.

The other possibility of course is that primary age child has given very detailed account of events omitting some vital brackets and it was in fact 7-(5+6*13).

BIDMAS/BODMAS the I is indices, the O is orders or sometimes "others" I've heard on occasion. Same thing.

P is parenthesis (lovely word!)

@wineymummy -
So first, multiply 6 X 13 to get 65.
Then next addition. 5+65= 80.
Finally, subtraction. So 7-80 = -73

Just going with your logic here - I know you corrected the multiplication.

6 x 13 = 78, not 65
The 7 is +7
the 5 is -5

So it's +7-5+78

(-5+78) = +73

+7 + 73 = 80

Not going to get into a Mumsnet argument anyway but in my mind it makes sense and I don't think the teacher deserves a bashing.

It may make sense in your mind but it is wrong. And yes, the teacher deserves a bashing. This is basic maths. If she doesn't understand this, she shouldn't be teaching.

wine
BODMAS is not the rule, it is a mnemonic to help remember the rule. However it can cause confusion, as with the OP's teacher.

Consider adding 9, 2 and -3.
Whatever way you do it you want the same answer.
So -3+9+2 is the same as +9-3+2 or +9+2-3

I was shouted at on my phonics thread yesterday for suggesting that maybe possibly some teachers might not be maths literate enough to understand statistical analysis of research data...

Give the teacher the benefit of the doubt- we all make mistakes on things that we know about.
If the teacher got one calculation wrong that doesn’t mean she doesn’t know how to do it correctly at all. Maybe she got distracted halfway through.
My advice - tell the kid to take it back to the teacher with the answer 80 and see what se says

As a secondary maths teacher this really clarifies why I have various kids arriving at secondary school believing that you have to do addition before subtraction, and ending up with the same wrong answers as OP's son. If (some) primary teachers are teaching it wrong it makes much more sense than a load of pretty clever kids all making the same mistake over something that's been taught correctly. Please note, I do not think this is a problem with all primary teachers, or even very many primary teachers, but if half my class have had a teacher who suffers from this misconception (and won't even realise it, as they think they've remembered it correctly) then I have to unteach (probably not a real word, but you know what I mean) the misconception before I can then teach the right method.

The correct answer is 80 btw if anyone is still unsure.

7-5+6x13
7-5+78
2+78
80

Addition and subtraction have the SAME importance so it makes sense to do them in the order they're written.

I can see the logic with the -76 but its wrong (nother mathematician here).

@Overstating you'll have fun explaining that to the teacher though....

You can do addition before subtraction but in this case you'd be doing -5+78 which is 73 NOT 5+78 and then subtracting it. Its the most common mistake that the students in my classes make - that is - 5 NOT 5 as a starting point

I think if you think of terms of "doing things" to the numbers, then it is going to get confusing.

It's not a complicated sum, there aren't any brackets, just think of it listing what things are there:
6 piles of 13 things.
7 things,
5 holes where things are missing.

Then just add them all together:
80
Whatever way you add them together, it doesn't matter, what's there is what's there.
You do have to chunk it up correctly, and that's what BODMAS helps you to do.
Equally you could just remember that you can't leave a loose "x" sign floating about, it always has to be something x something.

@CuckooCuckooClock - Give the teacher the benefit of the doubt- we all make mistakes on things that we know about.

That is so basic that it would be really difficult to get it wrong, provided you understand it. I can't see how anyone who understands the rules could make that mistake.

It’s 80 (maths teacher).

It is sometimes called BODMAS but no one ever seems to know what the O stands for 😂
It could stand for Order, as after brackets the next thing you’d do in a calculation is powers (order/ index). One PP said the O stood for Of which makes no mathematical sense ever but lots of people have been taught this.

BIDMAS makes much more sense and all the schools I’ve ever taught in teach in maths. When year 7s start I teach BIDMAS but many of them have been taught BODMAS. I don’t tell them it’s wrong but explain that we generally use BIDMAS as everyone is clear what the letters stand for!!

Well this just proves that ALL primary teachers are empathetic people’s people, therefor arty and unable to do maths

#Runs for cover

@ChocolateTea - You can do addition before subtraction but in this case you'd be doing -5+78

Exactly. You could always look on it as if subtraction doesn't exist so rather than subtracting, you're adding a negative number. So it's 7 + (-5) + (6x13)

This is the way I explain Bidmas:
B
I
DM
AS
Add and subtract are equal.
The answer is 80.
(Gavel)

*would not could

letsgo not all calculators give the correct answer.