On a journey across Mumbai, a train averages 50 km per hour for 50% of the distance, 40 km per hour for 40% of it and 20 km per hour for the remaining. The average speed (in km/hour) for the whole journey is:

Average speed = Total distance / Total time. Let total distance = D. Time for each part: first 50% at 50 kmph = 0.5D/50 = D/100 hours. Next 40% at 40 kmph = 0.4D/40 = D/100 hours. Remaining 10% at 20 kmph = 0.1D/20 = D/200 hours. Total time = D/100 + D/100 + D/200 = 5D/200 = D/40. Average speed = D / (D/40) = 40 kmph. Option B is correct. Option A (42) and D (45) incorrectly weight the speeds.