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 do not have any balls.

Hollywn

Hollywn

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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 do not have any balls.

Answer & Explanation

Dabbaghnn

Dabbaghnn

Beginner2022-08-21Added 6 answers

Define indicator random variables Ii, i=1,...., 5 that marks whether ith box is empty or not. Observe that
P(Ii=1)=(1pi)10
since we have that ith box is empty if and only if all the balls have been put in remaining boxes and each ball is put in some other box rather that ith box with probability 1pi. Hence, the number of empty boxes can be expressed as N=i=15Ii, so, using the linearity of expectation, we get
E(N)=i=15E(Ii)=i=15P(Ii=1)=i=15(1pi)10

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