3. A bus travelling the first one third distance at a speed of 10 km/h, the next one third at 20 km/h and the last one-third at 60 km/h. The average speed of the bus is ?

3. A bus travelling the first one third distance at a speed of 10 km/h, the next one third at 20 km/h and the last one-third at 60 km/h. The average speed of the bus is ?

(a) 9 km/h

(b) 16 km/h

(c) 18 km/h

(d) 48 km/h

Answer :  (c) 18 km/h

\( \text{Speed}_{avg} = \frac{\text{Total distance}}{\text{Total time}} \)

\( = \frac{\frac{d}{3} + \frac{d}{3} + \frac{d}{3}}{\frac{\frac{d}{3}}{10} + \frac{\frac{d}{3}}{20} + \frac{\frac{d}{3}}{60}} \)

\( = \frac{3}{\frac{1}{10} + \frac{1}{20} + \frac{1}{60}} \)

\( = \frac{3}{\frac{6 + 3 + 1}{60}} \)

\( = \frac{180}{10} \)

\( = 18 \text{ km/h} \)