A car travels at 54 km/hr for first 20 seconds, 36 km/hr for next 30 seconds and finally 18 km/hr for next 10 seconds. Find its average speed. 

54 km/h=15m/s...(1)
36km/h=10m/s....(2)
18km/h= 5m/s.....(3)
(1)speed=s/t
15=s/20
300=s
(2)speed=s/t
10=s/30
300=s
(3)speed=s/t
5=s/10
50=s
average speed=total s/total t
=(300+300+50)/(20+30+10)
=650/60
=10.833m/s