Can someone explain how to get the answer for d/e?

With so many people working from home due to the pandemic, local internet providers are extremely busy supporting their customers. Tommy Trojan & Associates (TT&A) provides internet access to many homes in the LA area. On average, 60 customers call TT&A per hour. The inter-arrival times are exponentially distributed. The customer first waits on hold until one of TT&As 20 Phone Technicians is available. The Phone Technicians works with customers an average of 15 minutes (exponentially distributed) to resolve the customer issue.

It turns out that of the 60 requests coming into TT&A per hour (inter-arrival times are exponentially distributed), the Phone Tech Support agent solves 75% of the customer issues. The remainder require an onsite technician dispatched. There is typically an 8 hour wait time before the Onsite Technician arrives at the customers home. The technician works an average of 1 hour at the customer location and they resolve all customer issues they work on.

d) If the company target is to keep the onsite technicians busy 75% of the time, how many technicians does TT&A need to staff?

e) As noted in the original problem, the service time for a phone tech to work on problems was 15 min. Ignoring your answer to part (b), assume the wait time for a Phone Tech was 2 minutes (this may or may not be the answer to b). For the onsite work described above, recall the wait time was 8 hours and the average service time was 1 hour. What is the average time for all customers from initial call to resolution (in hours)?

It turns out that of the 60 requests coming into TT&A per hour (inter-arrival times are exponentially distributed), the Phone Tech Support agent solves 75% of the customer issues. The remainder require an onsite technician dispatched. There is typically an 8 hour wait time before the Onsite Technician arrives at the customer's home. The technician works an average of 1 hour at the customer location and they resolve all customer issues they work on. 25% Wait for Tech Phone Tech Support Wait for on-site Tech Onsite Tech Support Fixed 75% Fixed d) If the company target is to keep the onsite technicians busy 75% of the time, how many technicians does TT&A need to staff? 20 onsite techs e) As noted in the original problem, the service time for a phone tech to work on problems was 15 min. Ignoring your answer to part (b), assume the wait time for a Phone Tech was 2 minutes (this may or may not be the answer to b). For the onsite work described above, recall the wait time was 8 hours and the average service time was 1 hour. What is the average time for all customers from initial call to resolution (in hours)? 2.5 hrs