Suppose that the number of people who enter an elevator
on the ground floor is a Poisson random variable with mean 10.
If there are n floors above the ground floor
and if each person is equally likely to get off at any one of these n floors
independently of where the others get off,
computer the expected number of stops that the elevator will make
before discharging all of its passengers.
(Assumepeople only enter the elevator on the ground floor
and it has infinite capacity.)
我知道大概是用條件期望值
但推不出來
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