Solution 12
The Probability for waiting 0 seconds is 1/2 (green light).
Given that the light is red (probability =1/2) the probability that
you will have to wait for exactly t seconds, i. e. that the light changes
in the interval t<x<t+dt , is equal to dt/30 in the time interval
0<x<30 and zero outside of this. The expected time for waiting is
thus
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Last updated: 2001-10-30 by Ingvar
Jönsson