The probability that the colour changes, while she is watching, is 1/7
Concept: Event probability = number of favorite results /Total number of results
It is given that the traffic light runs repeatedly in the following time cycle: green for 30 seconds, then yellow at 3 seconds, and then red for 30 seconds.
This means that the traffic light passes through 63 seconds to complete one-time cycle.
It is also given that Leah chooses a random 3 second time interval to watch the light.
We should find the probability that the color changes while Leah is looking.
The green light will change after 30 seconds and after three seconds it will change to yellow light, that is at 33 second it is yellow and after another 30 seconds it will change to red light.
Now let time t=0
The light changes at three different times: t=30,t = 33 and t = 63
This means that the light will change if Leah watches at intervals [27,30]
, [30,33] or [60,63]
Since each interval is 3 seconds and there are 3 intervals, Leah will have a chance of 9 seconds.
That makes a total of 9 seconds out of 63
⇒Probability that Leah watches the color change=Number of seconds she watches/Total seconds
=9/63=17
∴ The probability that Leah observes the color change is 1/7
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