00:01
The commuter on the way to work encounters four traffic signals.
00:06
Each of the traffic signals is far away from the other ones that the probability of getting a green light is independent of what happened at previous intersections.
00:14
The first two lights are green for 40 seconds of each minute, the last two for 30 seconds of each minute.
00:20
And we are asked to find the probability that the commuter will have to stop at least three times.
00:27
Okay, so let's consider our event.
00:31
Commuter stops at least three times.
00:46
R &g are my lights, my red and green lights, and i, the ice is, ice light is red and green, and i can be one, two, three, four.
01:16
And also we have what, p times r, i, equals 1 minus p times g i okay i'm going to go to the next page so now we can go p r1 equals p r2 equals one third and p r3 equals p r4 equals one -half okay p e equals p stop exactly three times and p stop four times.
02:21
Ok, so p.
02:22
I'm going to have to pay attention now...