In my AP Government class in preparation of a quiz we are given an unlimited amount of attempts at a practice quiz. There is a bank of 40 questions. Each practice quiz consists of 10 random questions (no repeats) drawn from this question bank. The real quiz is just like a practice quiz and takes 10 random questions (again, no repeats) from the bank. If I take, say, 14 practice quizes, what is the probability that on the 15th real quiz I would receive a question I have not seen before (one that has never been on a previous practice quiz)?
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$\begingroup$ What have you tried? Certainly you're also taking AP Statistics concurrently? $\endgroup$– Parcly TaxelCommented Feb 28, 2019 at 5:46
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$\begingroup$ @ParclyTaxel I am actually not taking AP stats. I asked my calc teacher and he gave some advice but no solution. I wrote a computer program that simulates this senario which allowed me to experimentally find the probability, which was about 17% taking 14 practice quizzes and about 2.5% taking 20 practice quizzes. Do these numbers seem reasonable? $\endgroup$– Jacob GCommented Feb 28, 2019 at 6:07
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$\begingroup$ Looks reasonable; see my answer. $\endgroup$– Parcly TaxelCommented Feb 28, 2019 at 7:14
1 Answer
This answer is adapted from the first part of this other answer.
Without loss of generality, fix the 10 real questions that appear on the real quiz. The probability that $j$ specific real questions have not been seen after $k$ practices ($1\le j\le10$) is $$S_j=\binom{10}j\left(\frac{\binom{40-j}{10}}{\binom{40}{10}}\right)^k$$ Then by inclusion/exclusion the probability that at least one real question has been seen is $$\sum_{j=1}^{10}(-1)^{j+1}S_j$$ For $k=14$ the probability is $0.165970\dots$, and for $k=20$ it is $0.031333\dots$