To find the average speed for the complete journey, we need to calculate the total distance traveled and the total time taken.
Let's assume the total distance traveled is "d" km.
The first half of the distance is d/2 km, and the salesman drives at a constant speed of 80 km/h for this distance. So, the time taken for the first half is (d/2) / 80 = d/160 hours.
The second half of the distance is also d/2 km, and the salesman drives at a constant speed of 120 km/h for this distance. So, the time taken for the second half is (d/2) / 120 = d/240 hours.
The total time taken for the complete journey is the sum of the time taken for the first half and the time taken for the second half:
Total time = d/160 + d/240 = (3d + 2d) / (3 * 160) = 5d / 480 = d / 96 hours.
The average speed is calculated by dividing the total distance traveled by the total time taken:
Average speed = Total distance / Total time = d / (d/96) = 96 km/h.
Therefore, the correct answer is D) none of these, as the salesman's average speed for the complete journey is 96 km/h.