Question

I am so lost in my honors algebra I class I am doing D R T (distance= rate * time)problems and need help the questions are similar to this one a ship must average 22 knots (nautical miles per hour) to make its 10-hour run on schedule. During the first four hours bad weather caused it to reduce speed to 16 knots. What should its average speed be for the rest of the trip to maintain its schedule?