Araft was sent away from point a downstream. a patrol-boat left a 5 hour and 20 min later and caught up with the raft 20 km away from point a. what was the speed of the raft if the speed of the patrol-boat was 12 km/hour?

Let's let x = speed of the raft 
Here is what we know:
distance = rate x time
distance traveled by both water vehicles is 20 km

To travel 20 km the patrol-boat will need how much time?
20 = 12 x t
t = 1 hour and 40 minutes

Subtract this from 5 hours and 20 minutes to determine how long the raft has been in the water: 5 hours 20 minutes + 1 hour 40 minutes = 7 hours

Now use distance = rate x time
20 = x (7) 
20/7 = x

The raft is traveling about 2.9 km/hour i.e. it's speed