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Original poster

Another maths one for those who enjoy such things...

51 replies

ThumbWitchesAbroad · 17/02/2019 12:15

Helping my year 6 son with some maths revision and found this in the revision book under the "Order of Operations" revision:

84 - 5(15-3) +25 = ?

So I'd like to know what people get as an answer, as I'm not sure myself, since the book answer and I disagree!

OP posts:
Dermymc · 17/02/2019 12:30

BODMAS should really be
B
O
DM
AS

Brackets, powers and then multiply and divide do as they are written. They are commutative so it doesn't matter what order. Then add and subtract. Subtract is essentially adding a negative so they happen at the same time.

dootball · 17/02/2019 12:30

84-60+25 =/= 84-(60+25)

This is not true!

altiara · 17/02/2019 12:30

I got -1 using BODMAS
You have to do addition before subtraction so you get 84-85

Interested in this thread?

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Rolypolypie · 17/02/2019 12:31

Dootball explained it well. It's not 60, it's -60.

Danglingmod · 17/02/2019 12:32

You're wrong, Altiara.

Original poster
ThumbWitchesAbroad · 17/02/2019 12:32

TeentimesTwo - thank you, that explanation is the best and makes the most sense - I will remember it.
I am actually entirely capable at maths, just had a blind spot on this one for some reason! Probably age-related...

OP posts:
Al2O3 · 17/02/2019 12:34

When entering the formula into both my Casio fx-99 and also Excel I get 49 in both cases.

Original poster
ThumbWitchesAbroad · 17/02/2019 12:34

dootball: =/= means "does not equal" so it is correct.

OP posts:
Whitegrenache · 17/02/2019 12:34

I think it's -1 due to BODMAS

dementedpixie · 17/02/2019 12:35

I got -1 using BODMAS
You have to do addition before subtraction so you get 84-85

No you don't as if you do addition first you would be adding -60+25 = -35
84-35= 49

dootball · 17/02/2019 12:36

This means does not equal : ≠

dementedpixie · 17/02/2019 12:36

I think it's -1 due to BODMAS

It's not. Read the explanations

Original poster
ThumbWitchesAbroad · 17/02/2019 12:36

Yes, but if you don't have access to that symbol, then so does =/=

OP posts:
Quartz2208 · 17/02/2019 12:36

Just because addition is first in the mnemonic they have equal priority so that gets done left to right so it’s 49

Original poster
ThumbWitchesAbroad · 17/02/2019 12:37

not handwritten, of course!

OP posts:
sackrifice · 17/02/2019 12:37

I think it's -1 due to BODMAS

huh?

how are you calculating this?

dementedpixie · 17/02/2019 12:39

The + or - is attached to the number after it so it's +84 -60 +25 Or -60 +84 +25

HarveySchlumpfenburger · 17/02/2019 12:41

It’s commonly typed as =/= though, dootball. The OP wasn’t wrong to use that.

BambooBoobam · 17/02/2019 12:45

I’m crap at maths but am learning new things —my Y6 child is teaching me—

I got 49 but can see why some people think -1.

I thought you followed BODMAS rules until you had a simpler sum? ie 84-60+25

So you wouldn’t do 15-3=12, 5x12=60, 60+25= 85, 84-85= -1 as that’s over complicating it?

dootball · 17/02/2019 12:47

My apologies, i have only seen it written as =/.

Original poster
ThumbWitchesAbroad · 17/02/2019 12:48

Yes Bamboo - I was indeed overcomplicating it.
And also forgetting that the minus sign attaches to the number directly after it.

These are things that I can now pass on to said Y6 son to make sure HE remembers them too (he has enough trouble with BODMAS as it is, without me making it worse for him, even by accident Blush)

OP posts:
sackrifice · 17/02/2019 12:52

So you wouldn’t do 15-3=12, 5x12=60, 60+25= 85, 84-85= -1 as that’s over complicating it?

Its not overcomplicating it, it is doing it wrong.

You can't add 60+25 and take from 84, as the 60 is -60 not +60.

TeenTimesTwo · 17/02/2019 12:56

=/= can be used to write does not equal.

Or you could use

(In coding you would write != )

BambooBoobam · 17/02/2019 15:22

Glad I found this thread as I had no idea about the + or - being attached to the number after it. It makes much more sense now.

BrieAndChilli · 17/02/2019 15:28

You do
Anything in brackets first
Then any special things like squares, etc
Then multiply and divide - these are equal importance so you just go left to right
Then add and subtract - again these are equal so you just do them left to right.