A person covers a certain distance by car at a speed of 30 km/h and comes back at a speed of 40 km/h.…

The correct choice is: “34.3 km/h”

When the same distance is covered at two speeds x and y, the average speed is total distance ÷ total time, which equals 2xy/(x + y), not the simple average of the speeds.

Average speed = (2 × 30 × 40)/(30 + 40) = 2400/70 = 34.29 ≈ 34.3 km/h.

Check with a distance of 120 km: time out = 4 h, time back = 3 h; total 240 km in 7 h = 34.3 km/h.

“35 km/h” is the simple arithmetic mean (30 + 40)/2, which is wrong because more time is spent at the slower speed. “37.5 km/h” and “32.8 km/h” do not match the calculation.