5.8 An electronic signboard in a sub- way station displays W, the time (in whole
ID: 3254057 • Letter: 5
Question
5.8 An electronic signboard in a sub-
way station displays W, the time (in
whole minutes) since the last train ar-
rived. Note that the signboard only
displays integers: if the last train ar-
rived t = 3:02 minutes ago, the sign-
board displays W = btc = 3. The in-
stant a train arrives, the signboard re-
sets to W = 0. Curiously, the trains
follow a synchronized schedule: interar-
rival times are independent and are ex-
actly K minutes long such that K is a
discrete uniform (2; 5) random variable.
That is, PK(k) = 1=4 for k = 2; 3; 4; 5.
If you arrive at a random time, what
is the probability 3 that the signboard
says W = 3?
Explanation / Answer
W = 3 when the interarrival time is between 3 and 4 minutes
f(x) follows uniform (2,5)
P(3<x<4) = P(x<4) - P(x<3)
=> F(4) - F(3)
=>2/3 - 1/3
=> 1/3
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