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