Lame ass maths riddle

Mellor

Nadser wrote: »

Ok, know this thread is a bit old, but here's my input (degree in maths by the way)

The question is 30/2(2+3)/5

Re-write it as an algebraic problem

Let x=30, y=2, z=3, w=5
The problem becomes: x/y(y+z)/w
x/(y^2+yz)/w

Substitute in the values
30/(4+6)/5
30/10/5
3/5 = 0.6

Subbing in letters makes no difference if you make the same mistake with multiplying

twerg_85

Mellor wrote: »

Subbing in letters makes no difference if you make the same mistake with multiplying

Not sure about that, since the whole discussion deals with interpretation of symbols.

What is 30÷2a÷5 where a = 5. Is it different if we set a = (2+3) instead of 5?

Or more to the point, why is 2a defined as (2a) but 2(2+3) is defined as 2 * (2+3) instead of (2 * (2+3))which is causing the confusion.

F.

Mellor

twerg_85 wrote: »

Or more to the point, why is 2a defined as (2a) but 2(2+3) is defined as 2 * (2+3) instead of (2 * (2+3))which is causing the confusion.

Because 2a is one number. There is no operation between them. It's a single number. if a=5, then 2a=10

30/2a/5 is not the same as 30/2*a/5 (actually just solve that in terms of A and tell me what you get)

2(2+3) is (2)*(5)
At this point we have reduced the equation to multiplication and division and naw go back to the start and solve left to right.
By defining it as (2 * (2+3)) you are adding extra brackets, which changes everything.

It's written in a ridiculous vague manner on purpose. So people solve the (2+3) first (which is correct its the part in brackets), then the 2*(5) bit next (which is wrong). The brackets are there to throw you off.

Nadser

Sorry man, disagree with you there. Can you provide anything else to prove your point?

28064212

Nadser wrote: »

Sorry man, disagree with you there. Can you provide anything else to prove your point?

Only links needed

28064212 wrote: »

Order of operations => Do what's inside the brackets first. All the other operations are of equal precedence (multiplication and division), so we look at...
Operator associativity => multiplication and division are both left-associative, so are performed from left-to-right

If you can explain why the order of operations and operator associativity should be changed in the OP's equation, go ahead

Nadser

I was hoping for something a bit more authoritative than wikipedia.

Here's a link I found - http://mathforum.org/library/drmath/view/57021.html

Mellor

Nadser wrote: »

Sorry man, disagree with you there. Can you provide anything else to prove your point?

What bit do you disagree with? Care to provide a reason.

That adding brackets changing the equation?
That multiplications amd division goes from left to right equally?

As a further proof, you can convert all number and operations to multiplication, therefore removing the issue of order.
To do this, change the sight of the index, ie /2 becomes *2^-1

30/2(2+3)/5 becomes

30*2^-1*(2+3)*5^-1

30*2^-1*5*5^-1 The easiest way here is to cancel the 5s with each other leaving
30*2^-1 converting 30 to terms of 2 leaves
15*2*2^-1 Now the 2s cancel leaving
15

Or you could do it from left to right
30*2^-1*5*5^-1 .....................30*2^-1 cancels to 15 as above
15*5*5^-1
75*5^-1..........in terms of 5 gives
3*5^2*5^-1........merging like like terms so a single power
3*5^1...............which also equals
15

twerg_85

28064212 wrote: »

Only links needed


If you can explain why the order of operations and operator associativity should be changed in the OP's equation, go ahead

http://en.wikipedia.org/wiki/Multiplication
In algebra, multiplication involving variables is often written as a juxtaposition (e.g. xy for x times y or 5x for five times x). This notation can also be used for quantities that are surrounded by parentheses (e.g. 5(2) or (5)(2) for five times two).

Nothing to do with order of operations, more to do with whether 2(2+3) is intended to be one number, or as 2 numbers with an operator between them. The above (whilst of course not being authoritative) suggests some ambiguity.

In short, if you or Mellor tell me that there's 30/2(2+3)/5 euro waiting for me 10 minutes walk away, I'll go and collect the 15 quid. If anyone else on this thread tells me the same thing, I won't bother going to pick up my 60c.

Mellor

twerg_85 wrote: »

Nothing to do with order of operations, more to do with whether 2(2+3) is intended to be one number, or as 2 numbers with an operator between them. The above (whilst of course not being authoritative) suggests some ambiguity.

It isn't really order of operations, that was stated so people grasp why .6 was wrong. As my example above shows, they can be done in any order and you get 15, as long its its done correct.
Did you understand it above?

I think we can all agree that the issue comes from the /2(2+3) part.
The problem is how people are solving it. Multipling it into the brackets and arrivign at /10.

That's the mistake. If is was written as /(2(2+3)) that would be fine. But its not. There is no bracket between the / sign and the 2. The operation of / is attached to the 2 and it can't be removed (otherwise we could rearrange equations lots of ways).

/2*5 is how that part reads, and is equal to *5/2 and therefore is means times two and a half. We know this from primary school maths. The order can't change the answer.

/2*5 is a stupid way to write it and is only done to as to confuse you into makign the mistake of solving is as /10 - you are supposed to fk it up, its a joke, a riddle etc. Which should be a clue to the right answer

Antikythera

Mellor wrote: »

Because 2a is one number

2a actually means 2 multiplied by a.

In the same way, 2(2+3) means 2 multiplied by (2+3).

2(2+3) is a single entity.

Therefore the answer to the OPs question is 30/10/5 = 0.6


Any ambiguity in calculation comes not from any confusion over 2(2+3), but from the 30/10/5.

(30/10)/5 gives a different answer to 30/(10/5)

Mellor

LOL, why are you doing the 2(5) multiplication before anything else. retarded logic FTW

Mwalimu

nyarlothothep wrote: »

Here's another one

-20=-20
16-36=25-45 (16-36=-20,25-45=-20)
4^2-36 = 5^2-45 (4^2=16,5^2=25)
4^2-36 = 5^2-45
4^2-2.4.9/2 = 5^2-2.5.9/2 (2.4.9/2=36,2.5.9/2=45)
4^2-2.4.9/2 +(9/2)^2 = 5^2-2.5.9/2 +(9/2)^2 (adding both the sides (9/2)^2
[4-(9/2)]^2 = [5-(9/2)]^2 (let 4=a,9/2=b)
4-(9/2) = 5-(9/2)
4 = 5
2+2 = 5

And

10x^2-19x=-6 =?

When you re-arrange it like that, you end up getting the square root of a negative quantity, which needs careful handling. Just because (-1)^2 = (1)^2 it does not follow that -1 = 1 by removing the square on each side. When you take the square root, there are different possibilities epending on how you handle the +/- roots. You've made an assumption that the one possibility you selected is the 'correct' one.

Antikythera

Mellor wrote: »

LOL

LOL.

Mellor wrote: »

why are you doing the 2(5) multiplication before anything else.

Is this a question? If so, then the obvious answer is that it is a fundamental rule of mathematics to simplify brackets before doing anything else.

Mellor wrote: »

retarded logic

LOL.

Mellor wrote: »

FTW

Whatever.

Antikythera

Mellor wrote: »

It's written in a ridiculous vague manner on purpose. So people solve the (2+3) first (which is correct its the part in brackets), then the 2*(5) bit next (which is wrong). The brackets are there to throw you off.

After further analysis, it appears you are correct.

Hats off to you sir.

I got there by trying to win your argument instead of mine.

a/b(c+d) is not equal to a/(b(c+d))

The former simplifies (correctly) to ac/b+ad/b = (ac+ad)/b

The latter simplifies to a/(bc+bd) and is wrong.

im invisible

and just when i was comming around to your way of thinking...

Antikythera

im invisible wrote: »

and just when i was comming around to your way of thinking...

Lol!

If faced with eg: ab(c+d) I would never consider first calculating b(c+d)

It just wouldn't be sound.

I don't mind admitting when I'm wrong.

MJRS

Antikythera wrote: »

Mellor wrote: »

LOL

LOL.

This post made me laugh so much for some reason, glad this thread is over though. LOL!

skregs

Jesus, never seen so many autistic idiots arguing over the correct answer for a deliberately ambiguous question

Mwalimu

Dougls Adams was about right in his Hitchhiker's Guide to the Galaxy. 42 is the right answer; it's the question that's wrong.

Nadser

Mwalimu wrote: »

Dougls Adams was about right in his Hitchhiker's Guide to the Galaxy. 42 is the right answer; it's the question that's wrong.

Eh, question wasn't wrong - they just didn't know what the question was!