Author Topic: Probability  (Read 2462 times)

Offline cashem'up

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Probability
« on: May 05, 2011, 06:50:49 am »
Read the question then record your one choice in the separate answer booklet.
The weather is dreadful here. It rains on three times as many days as there are rain-free days.
Luckily the daily weather forecast is fairly good. Whether the forecast is for rain or for no rain, it is
correct on nine occasions out of ten.
Which one of the following is the probability that the weather forecast will predict rain tomorrow?

The answer is 13/16

Can someone explain how

Offline Nobody

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Re: Probability
« Reply #1 on: May 05, 2011, 08:05:49 am »
i'm a bit confused too, found something similar here. :)

http://www.physicsforums.com/showthread.php?t=268897
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Offline cashem'up

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Re: Probability
« Reply #2 on: May 05, 2011, 10:19:14 am »
yea i had looked up that... didnt make sense to me :( ... lets just see if someone realy awesome can do this...

Offline Arthur Bon Zavi

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Re: Probability
« Reply #3 on: May 05, 2011, 10:57:34 am »
Read the question then record your one choice in the separate answer booklet.
The weather is dreadful here. It rains on three times as many days as there are rain-free days.
Luckily the daily weather forecast is fairly good. Whether the forecast is for rain or for no rain, it is
correct on nine occasions out of ten.
Which one of the following is the probability that the weather forecast will predict rain tomorrow?

The answer is 13/16

Can someone explain how

What is that supposed to be meant ?

Can you provide with a snapshot of the paper or type in other options ?

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Alpha

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Re: Probability
« Reply #4 on: May 05, 2011, 11:00:28 am »
What is that supposed to be meant ?

Can you provide with a snapshot of the paper or type in other options ?

P (rain) = 3/ 4  :-\

http://www.xtremepapers.me/CIE/International%20A%20And%20AS%20Level/9694%20-%20Thinking%20Skills/9694_s07_qp_3.pdf

Offline cashem'up

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Re: Probability
« Reply #5 on: May 05, 2011, 11:42:00 am »
P (rain) = 3/ 4  :-\

http://www.xtremepapers.me/CIE/International%20A%20And%20AS%20Level/9694%20-%20Thinking%20Skills/9694_s07_qp_3.pdf

yea exactly that paper... i got as far as this
1. it rains - 3/4 --> Forecast true --> 9/10 = 27/40
2. it doesnt rain - 1/4 --> Forecast false --> 1/10 = 1/40

So probability tht rain is forecasted is 28/40 ??? dont know where I have faltered

Offline Arthur Bon Zavi

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Re: Probability
« Reply #6 on: May 05, 2011, 12:11:44 pm »
yea exactly that paper... i got as far as this
1. it rains - 3/4 --> Forecast true --> 9/10 = 27/40
2. it doesnt rain - 1/4 --> Forecast false --> 1/10 = 1/40

So probability tht rain is forecasted is 28/40 ??? dont know where I have faltered

No.

(i) - It rains and predicts (3/4 X 9/10) = 27/40

(ii) - It doesn't rain and predicts (1/4 X 9/10) = 9/40

(i) + (ii) : 36/40 = 9/10
« Last Edit: May 06, 2011, 05:31:08 am by Fidato »

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Offline cashem'up

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Re: Probability
« Reply #7 on: May 13, 2011, 06:43:32 am »
No.

(i) - It rains and predicts (3/4 X 9/10) = 27/40

(ii) - It doesn't rain and predicts (1/4 X 9/10) = 9/40

(i) + (ii) : 36/40 = 9/10

The answer is 13/16 ... how in the world...

Offline Arthur Bon Zavi

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Re: Probability
« Reply #8 on: May 13, 2011, 08:10:04 am »
The answer is 13/16 ... how in the world...

CIE doesn't know to make a silly mistake. Leave it.

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Alpha

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Re: Probability
« Reply #9 on: May 13, 2011, 09:40:18 am »
The answer is good. We just did not study thinking skills. I never even knew about the existence of such a subject.

Offline Dasith

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Re: Probability
« Reply #10 on: May 13, 2011, 02:43:13 pm »
technically speaking this sum is meaningless!

it says it rains as many days as there are rain free days:

so lets take rainy days as 30
             dry days ;)as 10

Its obvious that the whether forcasting thing would predit;it would rain
30 days out of the total 40 days.(not less than dat)

but it says out of there is 1/10 chance that they are wrong :O
so out of the 30 days:
 30 x 1/10 of days could be wrong(i.e it wouldn't rain if they say it would)
 so 3 days are gone dry
since it rains 30 days anyway 3 days should rain !!

out of the 10 dry days:
 10x9/10 is dry so 9 days are dry :
 10 x 1/10 of days could be wrong(i.e it would rain if they say it wouldn't)
 only one day could rain here ! i.e for prediction to be wrong
but theres 3 left!!

 

just correct me if i hav understood the sum wrong ,or if i'm wrong
in my calculations :D because i havent done math
I would love to change the world, but they won't give me the source code. ;)

Offline EMO123

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Re: Probability
« Reply #11 on: May 13, 2011, 03:00:29 pm »
ya fidato is correct i also did like that