It takes 6+4= 10 hour for the bus to complete the journey.

6 hours 6/ 10= 3/5 of the whole journey.

So the truck ran away 1-3/5=2/5.

So the truck speed is (2/5)/(3/5)=2/3 of the bus speed.

The bus walked the whole journey, and the truck walked two-thirds of the whole journey.

So the whole journey: 200/( 1-2/3)=600 km.