A motorboat left a harbor and traveled to an island at an average rate of 11 knots. The average speed on the return trip was 10 knots. If the total trip took 4.2 hours, how far is the harbor from the island?

since distance = time*speed,

11x = 10(4.2-x)
x = 2
So, the boat's speed is 2 knots. I expect now you can figure the distance.