Total time in process $>Tdu=\frac{R}{R}p$

Dominic runs an appliance repair shop and sells replacement parts for appliances to walk in customers. Customers take an average of $5$ minutes. It is a single phase system with $1$ server. The coefficient of arrivals and the coefficient of processing times is $1\mathrm{.}0.$ If the total time a customer is in the system is $10$ minutes, what is the probability that a customer will have more than $15$ minutes in the system?

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