A state department of transportation district is responsible for 300 miles of highway. During a winter storm, salt trucks spread an average of 400 pounds of salt per mile and travel at an average speed of 25 mph. Because of nonproductive travel time, the average efficiency of the trucks (i.e., the productive time spent salting the roads) is 60 percent. How many seven-ton trucks will be needed to complete the process of salting all roads within two hours?