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

[ThumbWitchesAbroad]

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!

49

I make it 49.

49

But if you apply the laws of BODMAS, you should do the addition prior to the subtraction, which would give an answer of -1

Do the bracket first so (15-3) = 12
Do the multiplication next so 5x12 = 60
84- 60 +25 = 49

No brackets first. B = brackets

Again though, why do the subtraction first?

Addition and subtraction are of equal importance so once you have done the brackets, etc you work your way from left to right to get the answer

Obviously brackets first.

what is your interpretation?

If you are doing addition first, what are you adding to the 25?

Plus you get the same answer whether you do the addition or subtraction first. You need to do brackets and multiply before doing adding/ subtracting

You need to do brackets and multiply first giving 84-60+25

Even if you changed the order and did 84 + 25 - 60 it would still give the same answer though

OK demented - I'm assuming you are knowledgable in this area - why do you switch to linear maths after doing the brackets then?
This is not an arsey response, it is a genuine question

84+25-60 gives the same answer = 49

The answer is 49, because it just is, but I can see what you mean ...

Brackets then multiplication give 60, then add 25 is 85. So 84-85=-1.

I know that's not right, but I can't explain why using BIDMAS!

You need to multiply 5 x the bracket answer (12) = 60 before adding /subtracting

Ah. Just fallen in to where my error is, it's ok.

I was creating another set of brackets where none should have been - an error that my son also did on a linear maths question where everything was subtracted.

Thanks all, no problem here.

It doesn't matter which order you do subtraction or addition, the answer is always the same. Just think of a number line.

You don't have to , but it easier too.

If you try to stick to add before subtract then you can, but you must add the 25 to minus 60 to get minus 35.

Then 84 - 35 = 49

Yes, Roly - that's exactly what I did.

why do you switch to linear maths after doing the brackets then?

You don't. You switch after doing the multiplication of 5 times 12.

Which is after getting 12 by doing 15-3.

15-3 = 12

12 x 5 = 60

Then you have 84-60+24.

Or 84+24-60.

The + and the - have equal preference, and the sign only sticks to the number it is just in front of.

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

49

It's -60+25 not 60+25 though which gives -35 so your way would give 84 -35 = 49