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Probability Question

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Director
Director
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Status: Apply - Last Chance
Affiliations: IIT, Purdue, PhD, TauBetaPi
Joined: 17 Jul 2010
Posts: 693
Schools: Wharton, Sloan, Chicago, Haas
WE 1: 8 years in Oil&Gas
Followers: 14

Kudos [?]: 71 [0], given: 15

GMAT Tests User
Probability Question [#permalink] New post 06 Sep 2010, 17:50
An electronic clock shows time in hours and minutes. For example, 08:15 means that it is now a quarter past eight. If one looks at the clock at some time between 6:00 and 9:00, what is the probability that he will see the digit 8 ?

The answer to this question is 2/5 arrived thus: 72 minutes with 8 in display, a total of 180.

My question is if I added all the numbers in display between 600 and 900 (inclusive) then I get 181 total. For example
600-659 - 60 #s
700-759 - 60 #s
800-859 - 60 #s
900 - 1 #
Total 181 of which there are 6 in the first group, 6 in the second group and 60 in the third that have 8 in display. So should not the probability be 72/181?
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CEO
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Status: Nothing comes easy: neither do I want.
Joined: 12 Oct 2009
Posts: 2793
Location: Malaysia
Concentration: Technology, Entrepreneurship
Schools: ISB '15 (M)
GMAT 1: 670 Q49 V31
GMAT 2: 710 Q50 V35
Followers: 178

Kudos [?]: 948 [0], given: 235

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Re: Probability Question [#permalink] New post 06 Sep 2010, 18:00
mainhoon wrote:
An electronic clock shows time in hours and minutes. For example, 08:15 means that it is now a quarter past eight. If one looks at the clock at some time between 6:00 and 9:00, what is the probability that he will see the digit 8 ?

The answer to this question is 2/5 arrived thus: 72 minutes with 8 in display, a total of 180.

My question is if I added all the numbers in display between 600 and 900 (inclusive) then I get 181 total. For example
600-659 - 60 #s
700-759 - 60 #s
800-859 - 60 #s
900 - 1 #
Total 181 of which there are 6 in the first group, 6 in the second group and 60 in the third that have 8 in display. So should not the probability be 72/181?


IMO probability should be 72/180

600-659 - 60 #s
700-759 - 60 #s
800-859 - 60 #s
900 - 0 #

Read the question in red, it said time in between 6-9 that means we should exclude the case 9:00.
The person will start at 6:00 and end at 8:59. He does not look at the 9:00 value.
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670-to-710-a-long-journey-without-destination-still-happy-141642.html

Director
Director
avatar
Status: Apply - Last Chance
Affiliations: IIT, Purdue, PhD, TauBetaPi
Joined: 17 Jul 2010
Posts: 693
Schools: Wharton, Sloan, Chicago, Haas
WE 1: 8 years in Oil&Gas
Followers: 14

Kudos [?]: 71 [0], given: 15

GMAT Tests User
Re: Probability Question [#permalink] New post 06 Sep 2010, 18:22
Well gurpreetsingh, what I don't understand is if I said between X and Y (and meant neither is inclusive) I should treat both the same, meaning if I exclude Y I also exclude X. In this case I then should exclude 600 and 900 so the total is now 179, why retain 600 and exclude 900?
_________________

Consider kudos, they are good for health

CEO
CEO
User avatar
Status: Nothing comes easy: neither do I want.
Joined: 12 Oct 2009
Posts: 2793
Location: Malaysia
Concentration: Technology, Entrepreneurship
Schools: ISB '15 (M)
GMAT 1: 670 Q49 V31
GMAT 2: 710 Q50 V35
Followers: 178

Kudos [?]: 948 [0], given: 235

GMAT Tests User Reviews Badge
Re: Probability Question [#permalink] New post 06 Sep 2010, 21:56
mainhoon wrote:
Well gurpreetsingh, what I don't understand is if I said between X and Y (and meant neither is inclusive) I should treat both the same, meaning if I exclude Y I also exclude X. In this case I then should exclude 600 and 900 so the total is now 179, why retain 600 and exclude 900?


Yar, dont mind but I have seen in many of your posts, you give more emphasis on solving the question rather than learning from it I might be wrong but I cant resist myself pointing out something. :-D

This problem is little ambiguous and might not appear on the exam. If you read my previous post again I have clearly mentioned that
"Read the question in red, it said time in between 6-9 that means we should exclude the case 9:00.
The person will start at 6:00 and end at 8:59. He does not look at the 9:00 value."

He will start at 6:00:00 , at 6:00:01 ( here 01 are secs) he will still see 6:00 on the digital clock. This will be continued till 8:59:59. As he will look at the clock between 6:00 and 9:00, he still has 60 secs post 6:00 to consider 6:00 case.
This is the reason 6:00 is included and 9:00 is not.

PS: Don't mind for my harsh words, that's my way to motivate people. 8-)
If you are not able to understand anything, you are always welcome. But I advice you to spend some time,even an hour to know your doubt. Learning from own mistakes is the best way to learn.
_________________

Fight for your dreams :For all those who fear from Verbal- lets give it a fight

Money Saved is the Money Earned :)

Jo Bole So Nihaal , Sat Shri Akaal

:thanks Support GMAT Club by putting a GMAT Club badge on your blog/Facebook :thanks

Get the best GMAT Prep Resources with GMAT Club Premium Membership

Gmat test review :
670-to-710-a-long-journey-without-destination-still-happy-141642.html

Re: Probability Question   [#permalink] 06 Sep 2010, 21:56
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