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# Probability of a question coming up in an exam

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Joined: 05 Jul 2017
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Probability of a question coming up in an exam  [#permalink]

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05 Jul 2017, 09:39
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Hi,

First post so here goes. If an exam has three questions and they can be from a range of six subjects but not chosen twice, what is the chance that any one of those six subjects will be included in the three exam questions? So if the subjects are A,B,C,D,E, and F and the questions 1,2, and 3, Q1 could be any one of the six, Q2 any one of the remaining five subjects and so on. I've tried various methods giving a range of values so am not sure of any of them.

Cheers

Gus

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Re: Probability of a question coming up in an exam  [#permalink]

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05 Jul 2017, 17:50
Hi Gus,

If this is an actual GMAT practice question, then you should include the 5 answer choices. You will also likely receive more of a response if you post it in the appropriate sub-forum. For example, the Problem Solving forum can be found here:

https://gmatclub.com/forum/gmat-problem-solving-ps-140/

To answer this question, arguably the easiest way to do the work is to calculate the probability that a particular subject WON'T appear among the three questions. You can then take that probability and subtract it from 1 to learn the probability that it WILL appear. Since the prompt tells us that a subject cannot appear more than once, we can set up the following calculation:

(not in Q1)(not in Q2)(not in Q3) =
(5/6)(4/5)(3/4) =
60/120 =
1/2

So there's a 50% chance that the subject WON'T appear and there a 100 - 50 = 50% chance that it WILL appear.

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Re: Probability of a question coming up in an exam  [#permalink]

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06 Jul 2017, 04:36
GusTracy wrote:
Hi,

First post so here goes. If an exam has three questions and they can be from a range of six subjects but not chosen twice, what is the chance that any one of those six subjects will be included in the three exam questions? So if the subjects are A,B,C,D,E, and F and the questions 1,2, and 3, Q1 could be any one of the six, Q2 any one of the remaining five subjects and so on. I've tried various methods giving a range of values so am not sure of any of them.

Cheers

Gus

Please note that this is not a correct way to post questions on the forum. Please read carefully and follow our RULES OF POSTING. Thank you.

--== Message from the GMAT Club Team ==--

THERE IS LIKELY A BETTER DISCUSSION OF THIS EXACT QUESTION.
This discussion does not meet community quality standards. It has been retired.

If you would like to discuss this question please re-post it in the respective forum. Thank you!

To review the GMAT Club's Forums Posting Guidelines, please follow these links: Quantitative | Verbal Please note - we may remove posts that do not follow our posting guidelines. Thank you.

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Re: Probability of a question coming up in an exam   [#permalink] 06 Jul 2017, 04:36
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