A car travels the first one third of a certain distant with a speed of 10 km/hr, the next one third distant with a speed of 20 km/hr and the last one third distance with a speed of 60 km/hr. The average speed of the car for the whole journey is `18\ k m//h r` b. `24\ k m//h r` c. `30 k m//h r` d. `36\ k m//h r`