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Given that:

Probability that \(P\) execute the job = \(\frac{1}{3}\)
Probability that \(Q\) execute the job = \(\frac{2}{3}\)
Probability that \(R\) execute the job = \(\frac{3}{5}\)

Probability that anyone not finishing the report = \(\frac{2}{5}\)
=> Probability that anyone finish the report = 1 - \(\frac{2}{5}\) = \(\frac{3}{5}\)

Given that only \(P\) and \(Q\) will execute the job and finish the report.
So,\(R\) will not execute the job

Therefore Probability of \(R\) not executing the job = 1- \(\frac{3}{5}\) = \(\frac{2}{5}\)

Required probability = (P execute the job and finish report) x ( Q execute the job and finish the report) x (R not executing the job)
= \((\frac{1}{3} * \frac{3}{5}) ( \frac{2}{3} * \frac{3}{5}) (\frac{2}{5})\)
= \(\frac{1}{5} * \frac{2}{5} * \frac{2}{5}\)
= \(\frac{4}{125}\)
Answer = B
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I did go with C but the OA says B. So here is why it could be B.

P(E) for P and Q to complete their jobs and reports =
[(P(A)*P(B)*P(C) all completing the Job) * (P(A)*P(B) completing the Report *P(C) not completing the Report)] +
[(P(A)*P(B) completing the Job * P(C) not completing the Job ) * (P(A)*P(B) completing the Report *P(C) not completing the Report)].

We should remember that when C does not complete this job it also implies that he cannot complete his report.

Substituting the values we get ->[\((1/3*2/3*3/5)* (3/5*3/5*2/5) = 12/625\)] + [\((1/3*2/3*2/5)* (3/5*3/5*2/5) = 8/625\)] = \(20/625\) = \(4/125\)

Do let me know if my understanding is wrong.

Aditya.
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The reason why I calculated B and not C is the last part of the question: then what is the probability that only P and Q will complete their jobs and finish their report For me completing the job and finishing their report implies that R was not able to complete the job. Therefore, it is not necessary to take into account, that R could have completed the job but not the report.
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Solution



Given:
    • P, Q, and R each try to execute a job and create a report on it
    • The individual probabilities for their completion of the jobs are \(\frac{1}{3}\), \(\frac{2}{3}\), and \(\frac{3}{5}\)
    • The probability for any of them not to finish the report is \(\frac{2}{5}\)
      o Hence, the probability of finishing the report = \((1 – \frac{2}{5}) = \frac{3}{5}\)
    • One can write the report only after finishing the job

To find:
    • The probability that only P and Q will complete their jobs and finish their reports

Approach and Working:

it is given that, as per the favourable event, only P and Q will complete their jobs and finish their reports

    • The probability that P will execute the job and finish the report = \(\frac{1}{3} * \frac{3}{5} = \frac{1}{5}\)
    • The probability that Q will execute the job and finish the report = \(\frac{2}{3} * \frac{3}{5} = \frac{2}{5}\)

Now, as only P and Q will complete their jobs and finish their reports, it also means, there are two possibilities exist for R

    • Either R will not finish the job, and definitely not finish the report (as one cannot finish the report without executing the job)
      o Probability of this event = \((1 – \frac{3}{5}) * \frac{2}{5} = \frac{2}{5} * \frac{2}{5} = \frac{4}{25}\)
    • Or else, R will execute the job but will not finish the report
      o Probability of this event = \(\frac{3}{5} * \frac{2}{5} = \frac{6}{25}\)

So, we can say,
    • Probability (only P and Q will complete their jobs and finish their reports) =
    P (P completes job & finish report) AND P (Q completes job & finish report) AND [P (R does not complete the job & does not complete the report) OR P (R does complete the job & does not complete the report)
    = \(\frac{1}{5} * \frac{2}{5} * [\frac{4}{25} + \frac{6}{25}]\)
    = \(\frac{1}{5} * \frac{2}{5} * \frac{2}{5}\)
    = \(\frac{4}{125}\)

Hence, the correct answer is option B.

Answer: B

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Hi expert,

I can not understand why we need to consider the probability of R as the question asks to find the probability that only P and Q will complete their jobs and finish their reports.

Plz help.
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Hello expert, request you explain this how do we consider the probability

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BARUAH
Hi expert,

I can not understand why we need to consider the probability of R as the question asks to find the probability that only P and Q will complete their jobs and finish their reports.

Plz help.

Hey BARUAH,

If you read the question carefully, the favourable event is defined as "only P and Q will complete their jobs and finish their reports." It means we have to ensure that R cannot complete the job and finish the report.

For this reason we have to consider the probability of R.

Hope this answers your query. :-)
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Hello expert, request you explain this how do we consider the probability

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Hey Anshup,

Can you provide a little bit more details about you query? The answer we provided takes all the probability values given in the question only.
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Hello EgmatQuantExpert, thank you for the detailed explanation.

My mistake was to miss the probability of not finishing the report when R did not even finish his job. I thought that since we are told that one cannot even start his report if he didn't finish his job, hence there is no need to multiply 2/5 (1-3/5 - probability that R will not finish his job) with 2/5 (probability of not finishing a report). Please help me understand why we need to take into consideration the probability of not finishing a report when we know that R did jot finish his job, hence he cannot even start his report? Thanks a lot!
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Hello Bunuel

Could you please look at this question.
It seems that answer should be C.
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EgmatQuantExpert

Solution





Now, as only P and Q will complete their jobs and finish their reports, it also means, there are two possibilities exist for R

    • Either R will not finish the job, and definitely not finish the report (as one cannot finish the report without executing the job)
      o Probability of this event = \((1 – \frac{3}{5}) * \frac{2}{5} = \frac{2}{5} * \frac{2}{5} = \frac{4}{25}\)


Answer: B
to be honest , I don't understand why we have to multibly (r not finishing the job ) * (r not finishing the report ) in the first case ,since if he didn't finish the job he can't even write the report , so the event will stop there :?
by multiblying we make the probability smaller than its actual value :? :?
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EgmatQuantExpert

Solution





Now, as only P and Q will complete their jobs and finish their reports, it also means, there are two possibilities exist for R

    • Either R will not finish the job, and definitely not finish the report (as one cannot finish the report without executing the job)
      o Probability of this event = \((1 – \frac{3}{5}) * \frac{2}{5} = \frac{2}{5} * \frac{2}{5} = \frac{4}{25}\)


Answer: B
to be honest , I don't understand why we have to multibly (r not finishing the job ) * (r not finishing the report ) in the first case ,since if he didn't finish the job he can't even write the report , so the event will stop there :?
by multiblying we make the probability smaller than its actual value :? :?

Agree. Can any expert shed more light on this?
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EgmatQuantExpert
and in addition, why i think the ans B is impossible. If u draw a probability tree for R, (3/5)(2/5)+(3/5)(3/5)+(2/5)(2/5) does not equal to 1. Since u said its impossible for R to do report if job is not finished, so (3/5)(2/5)+(3/5)(3/5)+(2/5)(2/5)+(2/5)(3/5)=1 is not possible. so the probability tree should be: (3/5)(2/5)+(3/5)(3/5)+(2/5) =1.

can someone please clarify if my understanding is correct?
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EgmatQuantExpert

Solution



Given:
    • P, Q, and R each try to execute a job and create a report on it
    • The individual probabilities for their completion of the jobs are \(\frac{1}{3}\), \(\frac{2}{3}\), and \(\frac{3}{5}\)
    • The probability for any of them not to finish the report is \(\frac{2}{5}\)
      o Hence, the probability of finishing the report = \((1 – \frac{2}{5}) = \frac{3}{5}\)
    • One can write the report only after finishing the job

To find:
    • The probability that only P and Q will complete their jobs and finish their reports

Approach and Working:

it is given that, as per the favourable event, only P and Q will complete their jobs and finish their reports

    • The probability that P will execute the job and finish the report = \(\frac{1}{3} * \frac{3}{5} = \frac{1}{5}\)
    • The probability that Q will execute the job and finish the report = \(\frac{2}{3} * \frac{3}{5} = \frac{2}{5}\)

Now, as only P and Q will complete their jobs and finish their reports, it also means, there are two possibilities exist for R

    • Either R will not finish the job, and definitely not finish the report (as one cannot finish the report without executing the job)
      o Probability of this event = \((1 – \frac{3}{5}) * \frac{2}{5} = \frac{2}{5} * \frac{2}{5} = \frac{4}{25}\)
    • Or else, R will execute the job but will not finish the report
      o Probability of this event = \(\frac{3}{5} * \frac{2}{5} = \frac{6}{25}\)

So, we can say,
    • Probability (only P and Q will complete their jobs and finish their reports) =
    P (P completes job & finish report) AND P (Q completes job & finish report) AND [P (R does not complete the job & does not complete the report) OR P (R does complete the job & does not complete the report)
    = \(\frac{1}{5} * \frac{2}{5} * [\frac{4}{25} + \frac{6}{25}]\)
    = \(\frac{1}{5} * \frac{2}{5} * \frac{2}{5}\)
    = \(\frac{4}{125}\)

Hence, the correct answer is option B.

Answer: B



Could you kindly explain why did you multiply 2/5 as if it is sure he will not complete the report there is no need to multiply 2/5.
Thanks in advance.
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