Average speed as total distance over total time

Steps:

• Distance XY: 3 km/h × (20/60) h = 1 km
• Distance YZ: 4 km/h × (15/60) h = 1 km
• Total distance: 1 + 1 = 2 km
• Total time: 20/60 + 15/60 = 35/60 = 7/12 h; average speed = 2 ÷ (7/12) = 24/7 ≈ 3.4 km/h

Why B is correct:

• Average speed formula (total distance ÷ total time) gives exactly 24/7 km/h, or 3.4 km/h.

Why the others are wrong:

• A: Miscalculates total distance or time, yielding an underestimate.
• C: Uses arithmetic mean of speeds (3 + 4)/2 = 3.5, ignoring unequal times/distances.
• D: Adds speeds directly (3 + 4 = 7, halved or similar error), overestimating.

Final answer: B