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 :

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