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  #1  
Old 05.06.2015, 16:06
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A mathematical Friday Thread

It appears that some kids are getting their knickers in a twist because their GCSE maths exam was too hard and have taken to twitter about it - Daily Fail Link

You lot are a smart lot, so who here can solve the question in question:

Quote:
There are n sweets in a bag. Six of the sweets are orange. The rest of the sweets are yellow.

Hannah takes a sweet from the bag. She eats the sweet. Hannah then takes at random another sweet from the bag. She eats the sweet.

The probability that Hannah eats two orange sweets is 1/3.

Show that n²-n-90=0
I'm ashamed to say that my maths is not that hot these days and it took a couple of minutes for me to figure it out.
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  #2  
Old 05.06.2015, 16:22
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Re: A mathematical Friday Thread

Quote:
Emily Clarke added: 'My mum works for an accountancy company and it took 4 accountants 2hrs to answer the sweets Q. They have maths degrees.'
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  #3  
Old 05.06.2015, 16:22
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Re: A mathematical Friday Thread

N sweets.
A orange = 6
B rest = N-6

Probability of taking 2 orange sweets:

all possibilities - in how many ways can we draw two sweets one after another, assuming we can tell sweets apart even if the’re of the same colour:
n(n-1) - number of all possible sweets taking out, when order matters (it doesn’t but it is easier to count)
Basically at first you have a choice between N sweets, and then between N-1.

In how many ways can one select two orange ones?
6(6-1) = 30

Probability P = 30 / (n(n-1))
P is given as 1/3

So we have, after multiplying both sides by 3:
1 = 90 / (n(n-1))
Now multiply both sides of the equation by n(n-1)

(n(n-1)) = 90
n2 -n = 90

Last edited by yacek; 05.06.2015 at 16:24. Reason: typos
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  #4  
Old 05.06.2015, 16:27
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Re: A mathematical Friday Thread

10 sweets
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  #5  
Old 05.06.2015, 16:31
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Re: A mathematical Friday Thread

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10 sweets
OK, right, but where does the question ask you to solve for n?

You don't need to solve n to show that n²-n-90=0 based on the information given.

Oh, and write your working down, boy as my teacher used to say.
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  #6  
Old 05.06.2015, 16:36
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Re: A mathematical Friday Thread

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N sweets.
A orange = 6
B rest = N-6

Probability of taking 2 orange sweets:

all possibilities - in how many ways can we draw two sweets one after another, assuming we can tell sweets apart even if the’re of the same colour:
n(n-1) - number of all possible sweets taking out, when order matters (it doesn’t but it is easier to count)
Basically at first you have a choice between N sweets, and then between N-1.

In how many ways can one select two orange ones?
6(6-1) = 30

Probability P = 30 / (n(n-1))
P is given as 1/3

So we have, after multiplying both sides by 3:
1 = 90 / (n(n-1))
Now multiply both sides of the equation by n(n-1)

(n(n-1)) = 90
n2 -n = 90
Seems a bit of an overcomplicated way of doing a rather simple question:

P(Orange) = 6/n
P(2ndOrange) = 5/(n-1)

6/n x 5/(n-1) = 1/3
30/(n(n-1) = 1/3
n(n-1) = 90
n(squared) - n = 90
n(squared) - n - 90 = 0

To calculate n you just stick the a b c into the quadratic formula and you will get -9 or 10 so the answer is 10
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  #7  
Old 05.06.2015, 16:41
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Re: A mathematical Friday Thread

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10 sweets
question was to derive the equation, not solve it. zero points. fail.

EDIT: if it is any consolation, you have the same reading comprehension/mathematical ability as the average Daily Mail reader (see comments: http://www.dailymail.co.uk/news/arti....html#comments )
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Old 05.06.2015, 16:41
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Re: A mathematical Friday Thread

Would be interested how any forum member's 16 year olds cope with this question. The English GCSE 16 year olds seem to be in melt down because it wasn't like past paper questions.

God forbid you're made to apply your knowledge in unfamiliar situations!
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  #9  
Old 05.06.2015, 16:43
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Re: A mathematical Friday Thread

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question was to derive the equation, not solve it. zero points. fail.

EDIT: if it is any consolation, you have the same reading comprehension/mathematical ability as the average Daily Mail reader (see comments: http://www.dailymail.co.uk/news/arti....html#comments )
Nah, zero points is pretty harsh. He's only missing the last line of the derivation, after all:

"and since 10^2 - 10 - 90 = 0, it follows that n^2 - n - 90 = 0. Q.E.D."
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  #10  
Old 05.06.2015, 16:47
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Re: A mathematical Friday Thread

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question was to derive the equation, not solve it. zero points. fail.

EDIT: if it is any consolation, you have the same reading comprehension/mathematical ability as the average Daily Mail reader (see comments: http://www.dailymail.co.uk/news/arti....html#comments )
In fact the only useful answer is 8 - as that is the number of sweets remaining. However this then flags up that this can't possibly be a question from a UK maths paper. There is no way a fatty chub chub child in the UK is only going to have just 10 sweets in a bag unless this question is asked when the bag of sweets is already 90% eaten and/or the child can move onto a Mars bar and/or bottle of coke.
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  #11  
Old 05.06.2015, 16:48
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Re: A mathematical Friday Thread

i saw similar practice questions p.14 onwards here: http://www.ocr.org.uk/Images/83374-p...athematics.pdf

so it shouldn't be totally unfamiliar. i guess the multiplying out the probability for 2 events might have caught them out. perhaps the question could have walked them through it by breaking it down into parts.
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  #12  
Old 05.06.2015, 16:51
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Re: A mathematical Friday Thread

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In fact the only useful answer is 8 - as that is the number of sweets remaining. However this then flags up that this can't possibly be a question from a UK maths paper. There is no way a fatty chub chub child in the UK is only going to have just 10 sweets in a bag unless this question is asked when the bag of sweets is already 90% eaten and/or the child can move onto a Mars bar and/or bottle of coke.
when i read: "The probability that Hannah eats two orange sweets is 1/3." i thought, this is surely wrong as she will eat all of them. but then i realised it didn't say that she eats only two orange sweets and doesn't account for the likelihood she eats only 2 sweets because the headscarf wearing illegal immigrants who got free housing next door beats her up and takes the remainder of her sweets while working illegally and claiming benefits after having 2 kids during their teen years. and now after this incident still cannot be deported due to the pesky human rights act.
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  #13  
Old 05.06.2015, 16:52
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Re: A mathematical Friday Thread

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I'm ashamed to say that my maths is not that hot these days and it took a couple of minutes for me to figure it out.
It took me just 30 seconds to figure out that I'm not able to figure it out.

Just looking at this question makes me feel light-headed; I'm like 0mg!
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  #14  
Old 05.06.2015, 16:53
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Re: A mathematical Friday Thread

for those wanting more of a challenge, here is the GCSE question from Phil_MCR's exam paper:

"aliyah and amina are two lesbian illegal immigrants. assuming under the human rights act, they can claim free IVF treatment costing £3000 per attempt and the probability of success is 30% per attempt. they can attempt to have a treatment once every 6 months and no treatment for a year after a successful attempt (from start of treatment).

during this time they get free housing costing £2000 per month and ancillary support services, free transport etc. of £300 per week.

once a child is born, they will receive support of £200 per month for the first child and £150 per month for each subsequent child.

they want to have two chilren.

assuming that only one child is born at a time, calculate the expected cost to the tax payer until the youngest child reaches the age of 16.

(12 marks)"

i obviously missed my calling as an maths exam paper writer.

That's MathNut's Friday evening sorted!
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  #15  
Old 05.06.2015, 16:54
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Re: A mathematical Friday Thread

Took me longer to write this than figure out the answer. And I'm a touch-typist.

1/3 = (6/n) . (5/(n-1)) = 30 / (n(n-1)) => n²-n-90=0

About O level standard, I'd say.
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Old 05.06.2015, 16:56
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Re: A mathematical Friday Thread

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Oh, and write your working down, boy as my teacher used to say.
And apparently it is neither big nor clever to respond with "Why? Don't you know how to do it."
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  #17  
Old 05.06.2015, 16:58
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Re: A mathematical Friday Thread

I really miss the times of Friday threads about toilet flushing, smokers, bikes and dogs.
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  #18  
Old 05.06.2015, 17:07
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Re: A mathematical Friday Thread

The biggest problem I see with this question is that it doesn't explain what n is, but expects you to know the standard terminology. I hate that sort of thing, always did.

Would much prefer if it just asked you to work out how many sweets there were at the start,
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Old 05.06.2015, 17:12
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Re: A mathematical Friday Thread

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The biggest problem I see with this question is that it doesn't explain what n is, but expects you to know the standard terminology. I hate that sort of thing, always did.

Would much prefer if it just asked you to work out how many sweets there were at the start,
But it tells you in the first sentence.
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  #20  
Old 05.06.2015, 17:16
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Re: A mathematical Friday Thread

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The biggest problem I see with this question is that it doesn't explain what n is, but expects you to know the standard terminology. I hate that sort of thing, always did.

Would much prefer if it just asked you to work out how many sweets there were at the start,
'There are n sweets in a bag'

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