A bus covers the first half of a certain distance with speed v1 and the second half with a speed v2. The average speed during the whole journey is:

উত্তর দেখান ও ব্যাখ্যা

উত্তর: (B) 2v1v2 / (v1 + v2)

ব্যাখ্যা

Average speed = total distance / total time = 2d / (d/v1 + d/v2) = 2v1v2/(v1 + v2), the harmonic mean.