A car travels the first one-third of a certain distance with a speed of 10 `k m//h r` the next one-third distance with a speed of 20 `k m//h r` and the lat one-third distance with a speed of 60 `k m//h r` . 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`