It would help if you told us what the problem itself says! Are you saying that the boat goes upstream 120 miles in 20 hours and goes downstream 120 miles in 5 hours?
It should be easy for you to check it yourself. If the boat has a normal speed of 15 mph and the current is 9 mph, the actual speed of the boat upstream, relative to the shore, is 15- 9= 6 mph. In 20 hours, the boat will have gone 6(20)= 120 miles. Downstream, the actual speed, relative to the shore, is 15+9= 24 mph. In 5 hours the boat will have gone 5(24)= 120 mi. Looks good.