A bus covers three successive 3 km stretches at speed of 10 km/hr, 20 km/hr and 60 km/hr respectively. Its average speed over this distance is :

Answer: Option C

Solution

Answer: Option C

Solution:

$$\begin{array}{rl}& \text{Average speed }=\frac{\text{Total distance }}{\text{Total time}}\\ & \text{Average speed}=\frac{60\times 3}{\frac{60}{10}+\frac{60}{20}+\frac{60}{60}}\\ & \text{Average speed}=\frac{180\text{ km}}{\left(6+3+1\right)\text{ hr}}\\ & \text{Average speed}=\frac{180\text{ km}}{\text{10 hr}}\\ & \text{Average speed}=18\text{ km/hr}\end{array}$$
Short trick formula :
$$\left(\text{Average speed}=\frac{3xyz}{xy+yz+zx}\right)$$