The speed of the current in a river is 6mph. a ferry operator who works that part of the river is looking to buy a new boat for her business. every day, her route takes her 22.5 miles each way against the current and back to her dock, and she needs to make the trip in a total of 9 hours. she has a boat in mind, but she can only test it on a lake where there is no current at all. how fast must the boat go on the lake in order for it to serve the ferry operator's needs?