BIDMAS question - what's the right answer

[Overstating]

7-5+6x13?

Thanks!

It bothers me a lot that the cheapo one in my coffee table drawer gives the answer “104”.

It's all rubbish anyway because to make sure whoever is looking at it does it right you'd want to put brackets in anyway.

When programming I considered it 'best practice' to make intentions clear to future developers enhancing code by putting in brackets. Same for calculations in spreadsheets.

It means 7 - (all the other sums done first)

No, it really doesn't. By your logic, 7-2+5 would equal 0. In fact it equals 10 - it is the sum of 7, -2 and 5. You seem to be labouring under the misapprehension that addition comes before subtraction. It doesn't. They are treated equally. Similarly, multiplication and division are equal.

If a primary school teacher genuinely believes that the answer is -73 the local secondary schools are going to have to do some remedial maths teaching.

But teen the whole point of learning the order of operations is so we don't need superfluous brackets all over the place. The next step for this is in substituting in algebra, where we really don't want them to put in extra brackets as it becomes much more confusing.

Best practice in other disciplines (and I would never have put brackets into calculations like that when I was writing code, so obviously depends what code you are writing) isn't what drives us as mathematicians. Mathematicians use the language of maths, including the very basic order of operations, which is an accepted convention across the globe.

MsAwesome

I agree. I was being slightly flippant.

But I would use 'superfluous' brackets if they helped to make the meaning clear, or if less mathematically fluent people might be reading the whatever. The compiler should get rid of them anyway.

Occasionally a superfluous set of brackets makes life easier in the tea world. In this case though the aim is not to carry out a useful calculation for its own sake, but to teach a class of ten year olds how to use the BIDMAS rules - so adding extra brackets would destroy the entire object of the exercise.

Ooh. Why are brackets needed in the tea world?
Is that to display fancy teapots on shelves?

My daughter, who is at high school now (so does current maths) says -73 too. Lol. I got 80 with my 'knowledge' but I'm sure when we were at school you just did it in the order it came in the sum unless there was brackets.

@Looneytune253 - My daughter, who is at high school now (so does current maths) says -73 too.

That's worrying. There seems to be a lot of primary teachers teaching this incorrectly.

Sadly a typo/autocorrect for “real”, not a delicious maths/tea crossover.

I'm sure when we were at school you just did it in the order it came in the sum unless there were brackets.

Unless your maths teacher was totally useless, you really wouldn’t have been taught to do it like that. This isn’t a new development in mathematics.

Answer is 80,but how is anyone getting -73? I get having -76 if you get confused about the order, but I can't for the life of me see the 73?

looney You are misremembering (or you were doing it wrong at school!)

At a push I got a -74 ignoring anything I know about maths.

Where's the -73 coming from?

OP changed the wrong answer to -76

Read the thread...the op realised she’d made a calculation mistake as well as not applying the orders of operations correctly to get -73. She meant -76. It’s strange that Looneytune253’s daughter got -73 as well.

pixie sorry I actually missed that. But someone's daughter also got -73?

No maths credentials here,just a TA.

X posted

80

Definitely 80.

I had the same problem with a teacher marking my daughter's homework wrong when she was in Year 5 and making her do it again (using the wrong method).

My daughter was correct the first time - I know as I'd helped her the evening before (I have a maths degree). I was very polite when I approached the teacher - but she told me 'she was tired' when she marked it ! No apology !

This is how you wrongly get -76

7-5+6x13

6x13 = 78.

5+78 = 83.

7-83 = -76.

Some responses make me doubt myself. I got my maths degree in 1960, having won a state scholarship. The answer is 80.

temeraire I'm really disappointed not to learn something new about the tea world. And I agree that in the real world, depending on which part of the real world, I might well choose to add some superfluous brackets. But not in a maths lesson, particularly not in a maths lesson where we are focusing on the order of operations.

I love my Canadian education. We were taught:
Brackets
Exponents
Division/Multiplication in the order in which they occur
Addition/Subtraction in the order in which they occur

D/M are equal, A/S are equal. The teacher is a twit

MsAwesome I agree. If you are learning order of operations there is no point putting the brackets in. I was only mucking about really. Probably unwise on a thread where some people are confused enough already.