A car covers 4 successive 3 km stretches at the speed of 10 kmph, 20 kmph, 30 kmph and 60 kmph respectively. Its average speed is:

Distance is same in all the four stretches. Let the distance be 120 km (LCM of 10, 20, 30 and 60)

$\because$Time at 10 kmph speed  $=\frac{120}{10}=12\mathrm{hr}$

$\because$Time at 20 kmph speed  $=\frac{120}{20}=6\mathrm{hr}$

$\because$Time at 30 kmph speed $=\frac{120}{30}=4\mathrm{hr}$

$\because$Time at 60 kmph speed $=\frac{120}{60}=2\mathrm{hr}$

Total time$=12+6+4+2=24\mathrm{hrs}.$

Total distance$=120 \mathrm{x} 4=480 \mathrm{km}.$

$\because$$\text{ Average speed }=\frac{\text{ Total distance }}{\text{ Total time }}=\frac{480}{24}=20\mathrm{km}/\mathrm{h}\text{. }$