The officers of a high school senior class are planning to rent buses and vans for a class trip. Each bus can transport 63 ​students, requires 5 ​chaperones, and costs ​$1,400 to rent. Each van can transport 9 ​students, requires 1​ chaperone, and costs ​$100 to rent. Since there are 756 students in the senior class that may be eligible to go on the​ trip, the officers must plan to accommodate at least 756 students. Since only 70 parents have volunteered to serve as​ chaperones, the officers must plan to use at most 70 chaperones. How many vehicles of each type should the officers rent in order to minimize the transportation​ costs? What are the minimal transportation​ costs?