Janson's Department Store in​ Stark, Ohio, maintains a successful catalog sales department in which a clerk takes orders by telephone. If the clerk is occupied on one​line, incoming phone calls to the catalog department are answered automatically by a recording machine and asked to wait. As soon as the clerk is​ free, the party who has waited the longest is transferred and serviced first. Calls come in at a rate of about 10 per hour. The clerk can take an order in an average of 3.0 minutes. Calls tend to follow a Poisson​ distribution, and service times tend to be negative exponential.

The cost of the clerk is 12 per​ hour, but because of lost goodwill and​ sales, Janson's loses about 30 per hour of customer time spent waiting for the clerk to take an order.

​The average time that catalog customers must wait before their calls are transferred to the order clerk​ = ______ minutes ​(round your response to two decimal​ places).