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Can you Answer this maths question?

BblackkBbirdd
Posts: 919
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6/5/2015 12:38:16 PM
Posted: 1 year ago
This question was in the 2015 Edexcel maths paper, and a lot of people posted angry Tweets on the internet. I thought that the question was pretty easy, but I wanna see how you people do:

"There are n sweets in a bag. Six of the sweets are orange. The rest of the sweets are yellow. Hannah takes a random 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 n"-n-90=0."

(n' means n squared)
Diqiucun_Cunmin
Posts: 2,710
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6/6/2015 7:46:49 AM
Posted: 1 year ago
6/n * 5/(n-1) = 1/3
90 = n(n-1)
n^2 - n - 90 = 0

For a probability question, this doesn't even fall into the 'moderate difficulty' category, and I only took core maths at school...

I think the angry people just didn't study the chapter :P

BTW, to show a square with ASCII characters, type ^2.
The thing is, I hate relativism. I hate relativism more than I hate everything else, excepting, maybe, fibreglass powerboats... What it overlooks, to put it briefly and crudely, is the fixed structure of human nature. - Jerry Fodor

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Diqiucun_Cunmin
Posts: 2,710
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6/6/2015 10:00:39 AM
Posted: 1 year ago
Apparently, the problem has been making the rounds on Hong Kong websites and people are laughing at those complainers XD

One of the first comments was, 'you arts students keep saying maths is useless. Well, at least with maths, you can show off in front of foreigners!'
The thing is, I hate relativism. I hate relativism more than I hate everything else, excepting, maybe, fibreglass powerboats... What it overlooks, to put it briefly and crudely, is the fixed structure of human nature. - Jerry Fodor

Don't be a stat cynic:
http://www.debate.org...

Response to conservative views on deforestation:
http://www.debate.org...

Topics I'd like to debate (not debating ATM): http://tinyurl.com...
BblackkBbirdd
Posts: 919
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6/6/2015 3:12:15 PM
Posted: 1 year ago
At 6/6/2015 10:00:39 AM, Diqiucun_Cunmin wrote:
Apparently, the problem has been making the rounds on Hong Kong websites and people are laughing at those complainers XD

One of the first comments was, 'you arts students keep saying maths is useless. Well, at least with maths, you can show off in front of foreigners!'

To be honest, we were mocking the people who couldn't do the question too in maths today.
Porkloin
Posts: 53
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7/22/2015 4:24:23 PM
Posted: 1 year ago
I don't know about tree diagrams or plugging the knowns and variables into probability formulas, and it's been almost 40 years since I've had any math classes, but I feel people are making this harder than it really is - that if they would just think about it, it's no big trick.

We know we are working with small numbers here. There can't be too many yellows, or the probability of picking two oranges in a row will be far less than 1/3. If there's 6 of each, the probability will be less than 1/4, i.e. 1/2 for the first, and 5/11 for the second, so less than 1/4 for picking two oranges. Thus, there has to be less than 6 yellows.

(We could also look at "n." n can't be very big, or "n squared minus n minus 90" would be far above zero. Example: 20^2 - 20 - 90 = 290.)

If there are 5 yellows, then the probability of getting an orange one the first time is 6/11, and 5/10 or 1/2 for the second, so 6/22 = 3/11 for picking two oranges in a row, still less than 1/3, and thus we must reduce the number of yellows further.

If there are 4 yellows, then 6/10 or 3/5 for picking the first one as orange, and 5/9 for the second. 3/5 x 5/9 = 15/45 or 1/3. Bingo.

Thus, n = 10 and 10^2 = 100 and 100 - 10 - 90 = 0.