What is the average speed of each boat if boat B overtakes boat A at the first buoy, 4km from the start?

For the annual Breakwater sailboat race,sailboats start singly at 1minute intervals. Boat B is scheduled to start immediately followed by Boat A. The captain of Boat B feels that no matter how fast boat A travels his boat can sail 1km/h faster. What is the average speed of both boats if boat B overtakes Boat A at the first buoy which is 4km from the start?