1. **Stating the problem:** We need to compare two hairdressers using queueing models: the ex-hairdresser modeled by an M|D|1 queue with deterministic service time $B=\frac{1}{3}$ hour (20 minutes), and the young employee modeled by an M|M|1 queue with service rate $\mu=3$ customers per hour. The arrival rate $\lambda$ is 2.5 customers per hour for both. 2. **Understanding the service rate $\mu$ for the ex-hairdresser:** The service rate $\mu$ is the reciprocal of the average service time. Since the ex-hairdresser takes 20 minutes per customer, convert this to hours: $$B = 20 \text{ minutes} = \frac{20}{60} = \frac{1}{3} \text{ hour}$$ 3. **Calculating $\mu$ for the ex-hairdresser:** $$\mu = \frac{1}{B} = \frac{1}{\frac{1}{3}} = 3 \text{ customers per hour}$$ 4. **Summary:** The ex-hairdresser's service rate $\mu$ is 3 customers per hour, which matches the young employee's service rate. 5. **Additional notes:** - The M|D|1 model assumes deterministic service times (fixed at 20 minutes). - The M|M|1 model assumes exponential service times with rate $\mu=3$. - Both have the same arrival rate $\lambda=2.5$ customers per hour. This allows the hairdresser to compare queue performance metrics like average wait time and queue length for both employees.