Question

(a) A building has n floors numbered 1,2,...,n, plus a ground floor G. At the ground floor, m people get on the elevator together, and each gets off at a uniformly random one of the n floors (independently of everybody else). What is the expected number of floors the elevator stops at (not counting the ground floor)

Answer 1

Answer:

The expected number of floors the elevator stops at, not counting the ground floor is =

n*(1-(1-1/n)^m)

Step-by-step explanation:

Here, we want to know the expected number of floors the elevator stops at.

let X1,X2,X3,..Xn are indicator variable for which value =1 if at least one person stops on that floor otherwise value is 0

P(at least one person stops at floor Xj)=1-P(none of m people stops at floor j)

=1-(1-1/n)^m

here total number of floors on elevetor Stops X=X1+X2+X3+...+Xn

hence expected number of floors on elevetor Stops

E(X)=E(X1)+E(X2)+E(X3)...+E(Xn)

=(1-(1-1/n)^m )+(1-(1-1/n)^m )+(1-(1-1/n)^m )+(1-(1-1/n)^m )+..... n times

=n*(1-(1-1/n)^m)