Problem 63A
For a certain individual, the daily number of telephone calls (incoming or outgoing) has a Poisson distribution with mean 12. The length in time (in minutes) of each telephone call has an exponential distribution with mean 5 minutes.
The length of time of one telephone call is independent of the length of time of any other telephone call.
On a given day, this individual makes or receives 4 telephone calls. What is the probability that this person is on the telephone for more than half an hour?
Problem 63B
For a certain individual, the daily number of telephone calls (incoming or outgoing) has a Poisson distribution with mean 16. The length in time (in minutes) of each telephone call has an exponential distribution with mean 8 minutes.
The length of time of one telephone call is independent of the length of time of any other telephone call.
On a given day, this individual makes or receives 5 telephone calls. What is the probability that this person is on the telephone for more than 45 minutes?
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Answers

Answers can be found in this page.
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Could you please post the process of how you arrived at the answer?
Please explain how you arrived a this answer
You can follow the process explained in this page:
https://probabilityexam.wordpress.com/2011/06/09/evaluatingthegammarighttail/
can you help me how to solve this problem?