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Four people drawing five of seventeen balls overlap

Question

An urn holds nn balls numbered 11 to nn. Each of k≥2k\ge 2 people in turn draws rr balls (1≤r≤n1\le r\le n) without replacement, records them, and returns all of them to the urn before the next person draws. Let p(k,n,r)p(k,n,r) denote the probability that at least one ball is drawn by every one of the kk people. Compute p(4,17,5)p(4,17,5) to three decimal places.

Short answer

About 0.1230.123. Apply inclusion–exclusion to the set of balls shared by all kk draws. A fixed group of jj balls lies inside one person's draw with probability qj=(n−jr−j)/(nr)q_j=\binom{n-j}{r-j}\big/\binom{n}{r}, hence p=∑j=1r(−1)j+1(nj)qj kp=\sum_{j=1}^{r}(-1)^{j+1}\binom{n}{j}q_j^{\,k}. Plugging in k=4k=4, n=17n=17, r=5r=5 gives ≈0.1233\approx 0.1233, which a simulation confirms.

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