Problem Analysis:
We are given that a motorboat is going downstream and it overtakes a raft at a point A. After some time, the motorboat turns back and passes the raft at a distance l=6.0km from point A. We need to find the flow velocity of the river assuming the duty of the engine to be constant.

Key Given Information:
- The motorboat overtakes the raft at point A.
- Time taken by the motorboat to turn back and reach point A again = 60 minutes.
- Distance between the motorboat and the raft when the motorboat turns back = 6.0km.

Solution:
Let's assume the speed of the motorboat in still water as b km/hr and the speed of the river flow as r km/hr.

Step 1: Calculate the speed of the motorboat downstream
The speed of the motorboat downstream can be calculated using the formula:
Speed downstream = Speed in still water + Speed of the river flow

Step 2: Calculate the time taken by the motorboat to reach point A when going downstream
The time taken by the motorboat to reach point A when going downstream can be calculated using the formula:
Time downstream = Distance / Speed downstream

Step 3: Calculate the speed of the motorboat upstream
The speed of the motorboat upstream can be calculated using the formula:
Speed upstream = Speed in still water - Speed of the river flow

Step 4: Calculate the time taken by the motorboat to reach the raft when going upstream
The time taken by the motorboat to reach the raft when going upstream can be calculated using the formula:
Time upstream = Distance / Speed upstream

Step 5: Calculate the total time taken by the motorboat for the round trip
The total time taken by the motorboat for the round trip can be calculated using the formula:
Total time = Time downstream + Time upstream

Step 6: Calculate the speed of the river flow
The speed of the river flow can be calculated using the formula:
Speed of the river flow = (Distance - Speed downstream * Time downstream) / Time downstream

Step 7: Substitute the given values and calculate the speed of the river flow
Substitute the values in the formula obtained in step 6 and calculate the speed of the river flow.

Step 8: Final Answer
The speed of the river flow is the calculated speed obtained in step 7.

Conclusion:
The flow velocity of the river can be calculated by following the steps mentioned above. By substituting the given values in the appropriate formulas, we can determine the speed of the river flow.