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    I have this question which I'm finding hard to solve:
    Each of a large number of researchers, in different parts of the country, carries out a survey in which they question randomly chosen people in turn, until they find one who admits to smoking more than 20 cigarettes a day. As soon as each researcher hasn't found one such person, the total number of people is noted. In collating the results of the survey, it is found that on average 95% of the researchers had to ask at least 30 people. Use this figure to estimate the probability that one person at random would admit more than 20 cigarettes a day.

    What I think I have to do:
    95/100 is the probability of success, 30 is the number of trials. I should do 95/100^30*5/100, to get the probability which is just the probability for one trial.
    I don't get the right answer. The right answer is 0.0018.

    Thanks in advance...
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    (Original post by Yewdraconis)
    I have this question which I'm finding hard to solve:
    Each of a large number of researchers, in different parts of the country, carries out a survey in which they question randomly chosen people in turn, until they find one who admits to smoking more than 20 cigarettes a day. As soon as each researcher hasn't found one such person, the total number of people is noted. In collating the results of the survey, it is found that on average 95% of the researchers had to ask at least 30 people. Use this figure to estimate the probability that one person at random would admit more than 20 cigarettes a day.

    What I think I have to do:
    95/100 is the probability of success, 30 is the number of trials. I should do 95/100^30*5/100, to get the probability which is just the probability for one trial.
    I don't get the right answer. The right answer is 0.0018.

    Thanks in advance...
    95% of researchers had to ask at least 30 people, means 5% of researchers found someone who smoked 20 or more within the first 29 people.


    If we let p be the probability that a person smokes 20 or more, then

    0.05 = p +(1-p)p + ... + (1-p)^{28}p

    And solve for p. Hope you're familiar with GP.s

    Edit:

    Alternatively you can go with the 95% had to ask 30 or more, in which case we have:

    0.95 = (1-p)^{29}p+ (1-p)^{30}p +....
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    (Original post by ghostwalker)
    95% of researchers had to ask at least 30 people, means 5% of researchers found someone who smoked 20 or more within the first 29 people.


    If we let p be the probability that a person smokes 20 or more, then

    0.05 = p +(1-p)p + ... + (1-p)^{28}p

    And solve for p. Hope you're familiar with GP.s

    Edit:

    Alternatively you can go with the 95% had to ask 30 or more, in which case we have:

    0.95 = (1-p)^{29}p+ (1-p)^{30}p +....
    Thanks but I don't know to do the summation of this, I know it can be factorised to p(1+(1-p)...=1, but then p=1 and I don't know what to do next
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    (Original post by Sibghy)
    Thanks but I don't know to do the summation of this, I know it can be factorised to p(1+(1-p)...=1, but then p=1 and I don't know what to do next
    Can't make sense of that. If you don't know what a GP is then it's not possible for you to do the question.
 
 
 
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