One-third of a journey is covered at a speed of 56 km/hr, the next one-third of the journey is covered at a speed of 7 km/hr, and the remaining journey is covered at a speed of 14 km/hr. The average speed (in km/hr, rounded off to 1 decimal place) for the entire journey is:
12.9
Let the total journey be 3d km, so each one-third part is d km. Time taken for each part = d/56, d/7, and d/14 hours respectively.
Total time = d/56 + d/7 + d/14 = d(1 + 8 + 4)/56 = 13d/56 hours.
Average speed = Total distance / Total time = 3d / (13d/56) = (3 x 56)/13 = 168/13 = 12.9 km/hr (rounded to 1 decimal place).
Hence, the correct answer is 12.9.
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