maths problem

[rsvp]

have just seen this and would welcome thoughts from greater brains than mine!
4x4+4x4+4-4x4 = ?.......I got20 but has been a long time since I was at school.
My reasoning is .....
(4x4)+(4x4) = 32
32+4= 36
- (4x4) 16 therefore answer is 36-16 =20

Very happy to be proven wrong but would love to know why!!! thank you

[Prudie]

I might say 4 to that factor 4/2/2 is 8/2....

[factor-fiction]

Actually BODMAS seems to be coming back into favour again. The O is 'other'

[MarkRae]

> 4÷2÷2

:-)

The two operators are the same, so in this case you DO go from left to right. The answer is indeed 1.

[Mojo-Jo-Jo]

Fair enough Mark.
I got the answer right though. :p

[sherrardk]

Boo, it's like knowing where punctuation goes in a sentences - it just is the way it is. I can do the sum, and get 20, but I couldn't explain to you why it is correct, it is just the way things are.

[Wharton]

.....anyway.....
12 x 12 = 144
13 x 13 = 169
21 x 21 = 441
31 x 31 = 961

[factor-fiction]

I agree with Mark that 4÷2÷2=1, but I don't like seeing it set out without brackets because I would wonder whether that was what the author really intended.

[Mick-Talbot]

My 9 year old says the answer is 20
and there was me going to agree with Joe....

[Mojo-Jo-Jo]

I just shift the brackets down one place mentally Factor, so I would put them round the multiplications. It just helps me to separate things.

[B00]

lol, thanks guys, and thanks rsvp for an unexpected interesting question!

I do understand that there's a rule that needs to be applied, and it has an arty farty name, but I dont understand the need for a rule in the first place, i still stand by my argument that if you did the sum in the simplest way you'd get 320, and everyone else would get the same answer too.

But...

<<Is willing to concede that its kinda like some laws, I don't understand why we have them, I just accept we do, and shut up ;-)

[joeluke]

'I don't like seeing it set out without brackets because I would wonder whether that was what the author really intended'

Exactly the same could be said about the poser in the OP couldn't it

[factor-fiction]

Well joe, if it was set in a maths textbook in a chapter on Order of Operations I'd know what the questioner intended.

If a colleague in a finance office had scribbled down a calculation involving a few operations and no brackets which he wanted me to work out I'd go back and clarify what calculation was intended

[mike11111]

It might help those who are confused as to the need for mathematical rules to understand that whilst we tend to read from left to right the languages of those countries in which mathematics originated tend to read from right to left, hence the need for strict rules.

[Mojo-Jo-Jo]

By the way, I don't agree with Mark that I'm wrong, but he's always so abrupt with me I'd rather not engage him too often.
Having looked it up, Wikipedia agrees with me, as do a couple of other sites.

http://en.wikipedia.o...i/Order_of_operations

[joeluke]

Exactly factor, you can't calculate the answer of a sum just on the basis of assumptions

[ChuckFickens]

"you can't calculate the answer of a sum just on the basis of assumptions"

They are not assumptions, they are rules, with the absence of any other information there is only one correct way of doing it.

[factor-fiction]

Fair enough. The answer to the problem is "if the problem has been set out correctly the answer is 20"

[joeluke]

'The answer to the problem is "if the problem has been set out correctly the answer is 20"'

'For 'if the problem' read 'if brackets had been inserted'

[Mojo-Jo-Jo]

The problem HAS been set out correctly. Follow the rules and you can't go wrong.

[factor-fiction]

No.