Hannah's Horseback-Riding is a new horseback riding company in Heber, Utah. The company prides itself on simplified operations. It does not accept reservations and the customers can simply walk in and rent one of the available horses. The company has a total of 20 horses. (Of course, at any given point in time, some of these horses will be rented away whereas others will be available for rent to an arriving customer.) On average, a customer arrives every 6 mins. The coefficient of variation of inter-arrival times is 1, i.e., CVa=1. Customers keep their horse for an average of 100 mins. The standard deviation of this time is 30 mins. On average, how many horses are available for rent