Probability

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800GMAT: Master | Next Rank: 500 Posts; Posts: 174; Joined: Mon Mar 26, 2007 5:51 am; Thanked: 1 times

by 800GMAT » Mon Nov 05, 2007 6:36 pm

There is a 10% chance that it won't snow all winter long. There is a 20% chance that schools will not be closed all winter long. What is the greatest possible probability that it will snow and schools will be closed during the winter?

(A) 55%
(B) 60%
(C) 70%
(D) 72%
(E) 80%

OA: E

need an approach to solve this one...

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Source: Beat The GMAT — Problem Solving | Mon Nov 05, 2007 6:36 pm

camitava: Legendary Member; Posts: 645; Joined: Wed Sep 05, 2007 4:37 amFollowed by: 5 members; Location: India; Thanked: 34 times

by camitava » Mon Nov 05, 2007 8:38 pm

800GMAT,
10% chance that it won't snow all winter long => 90% chance that it will snow all winter long
20% chance that schools will not be closed all winter long => 80% chance that schools will be closed all winter long
Combining these two statements together, we can get that there is 80% chance that schools will be closed all winter long and it will snow all winter long.
Am I clear 800GMAT?

Correct me If I am wrong

Regards,

Amitava

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samirpandeyit62: Master | Next Rank: 500 Posts; Posts: 460; Joined: Sun Mar 25, 2007 7:42 am; Thanked: 27 times

by samirpandeyit62 » Mon Nov 05, 2007 9:49 pm

This one seems to be a little weird to me

I agree that the max P that schools will be closed all winter long = 8/10 =80%

but if this is the P i.e the greatest possible probability that it will snow and schools will be closed during the winter.

then it means taking the P that it will snow = 1 i.e maximizing it

now if this corresponds to snow fall on atleast 1 day during winter (question mentions during winter & not all winter)

then the P =1 can be justified but in that case same will be applicable for the schools also so that too must considered as P of schools remaining closed on at least 1 day during winter, this P would be greater than 80%

so IMO the question has some logical flaw.

Any opinions/ corrections appreciated.

Regards
Samir

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yuri: Senior | Next Rank: 100 Posts; Posts: 45; Joined: Sun Oct 07, 2007 3:54 pm; Location: Seattle, WA; Thanked: 1 times

by yuri » Wed Nov 07, 2007 12:35 am

aren't we supposed to multiply 9/10 and 8/10 to get 72% of the both being closed/snowing together??

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samirpandeyit62: Master | Next Rank: 500 Posts; Posts: 460; Joined: Sun Mar 25, 2007 7:42 am; Thanked: 27 times

by samirpandeyit62 » Wed Nov 07, 2007 4:05 am

Hi Yuri,
I also meant that 72% is the answer which comes out, for E to be correct, the probability of snow should be 1 which produces a conflict here.

Regards
Samir

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