Answer: To find the average speed, we need to use the formula:
average speed = total distance / total time
We can first calculate the total distance, which is:
total distance = 900 km + 3000 km + 400 km + 15 km = 3315 km
To calculate the total time, we need to find the time taken for each leg of the journey:
time taken for train journey = distance / speed = 900 km / 60 km/hour = 15 hours
time taken for boat journey = distance / speed = 3000 km / 25 km/hour = 120 hours
time taken for plane journey = distance / speed = 400 km / 350 km/hour = 1.14 hours
time taken for taxi journey = distance / speed = 15 km / 25 km/hour = 0.6 hours
Therefore, the total time taken for the entire journey is:
total time = 15 hours + 120 hours + 1.14 hours + 0.6 hours = 137.74 hours
Now we can calculate the average speed:
average speed = total distance / total time = 3315 km / 137.74 hours ≈ 24.05 km/hour
Therefore, the average speed for the entire distance is approximately 24.05 km/hour.