Final answer:
The man's average speed for the whole journey is calculated by dividing the total distance (100 km) by the total time taken (1 hr), which results in an average speed of 100 km/hr, option (c).
Step-by-step explanation:
The average speed for the entire journey can be found using the formula for average speed, which is the total distance traveled divided by the total time taken. For the first part of the journey, the man covers 20 km at a speed of 40 km/hr, calculated as follows:
Total time for first part = Distance / Speed = 20 km / 40 km/hr = 0.5 hrs
For the second part, he covers 80 km at a speed of 160 km/hr, which is calculated as:
Total time for second part = Distance / Speed = 80 km / 160 km/hr = 0.5 hrs
The total distance covered is the sum of the two distances: 20 km + 80 km = 100 km. The total time taken is the sum of the two times: 0.5 hr + 0.5hr = 1 hr.
Now we can calculate the average speed for the entire journey:
Average speed = Total distance / Total time = 100 km / 1 hr = 100 km/hr
So the man's average speed for the whole journey is 100 km/hr, which is the correct answer (c).