Help with math question!!!?

The Yankee Clipper leaves the pier at 9:00 AM at 8 knots (nautical miles per hour). A half hour later, The River Rover leaves the same pier in the same direction traveling at 10 knots. At what time will The River Rover overtake The Yankee Clipper? Could you please solve this and tell me how you did it. Our teacher told us that for these type of problems use the formula d=rt, where d is distance, r is the rate, and t is the time.