Suppose that 10 balls are put into 5 boxes, with each ball independently being put in box i with probability p_i, sum_(i=1)^5 p_i=1 Find the expected number of boxes that have exactly 1 ball.

luxlivinglm

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2022-08-20

Suppose that 10 balls are put into 5 boxes, with each ball independently being put in box i with probability
pi,i=15pi=1
Find the expected number of boxes that have exactly 1 ball.

Answer & Explanation

Allyson Vance

Allyson Vance

Beginner2022-08-21Added 14 answers

Define indicator random variables Ji, i=1,...,5 that marks whether ith box has one and only one ball or not.
Observe that
P(Ji=1)=(beg{array}{c}101end{array})pi(1pi)9
since we have that ith box has one and only if we choose one ball out of ten and put it into that box, and all other balls we put in some of remaining boxes. Hence, the number of empty boxes can be expressed as M=i=15Ji, so, using the linearity of expectation, we get
E(M)=i=15E(Ji)=i=15P(Ji=1)=i=1510pi(1pi)9

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