Final answer:
The boat will take 1.5 hours to complete the trip.
Step-by-step explanation:
To find the time taken to complete the trip, we need to determine the speed of the boat when going upstream and downstream.
When a boat is going upstream, its effective speed is the difference between the speed of the boat in still water and the speed of the stream. So, the boat's speed upstream is 16 mph - 4 mph = 12 mph.
When the boat is halfway between city A and city B, it has traveled half the distance, which is 36 miles ÷ 2 = 18 miles. The time taken to travel this distance at a speed of 12 mph is 18 miles ÷ 12 mph = 1.5 hours. Therefore, it will take 1.5 hours to complete the trip.