A truck driver has to cover a total distance of 260 km in 4 hours. He drives at a speed of 56 km/h for the first 75 minutes, and the next 90 km at a speed of 60 km/h. At what speed (in km/h) must he drive the remaining distance in order to complete the journey in 4 hours?

Given, 

Total distance = 260 km

Total time = 4 hrs

Case I : 

Speed = 56 km/hr

Time = 75 min = 1.25 hrs

We know, Distance = $$speed\times\ time$$ 

= $$56\times1.25=70\ km$$

Case II : 

Speed = 60 km/hr

Distance covered = 90 km

tiime = $$\frac{dis\tan ce}{speed}=\frac{90}{60}=1.5\ hr$$

Remaining distance = $$260-\left(70+90\right)=100\ km$$

Remaining time = $$4-\left(1.25+1.5\right)=1.25\ hrs$$

So, speed must he drive the remaining distance in order to complete the journey in 4 hours be : 

speed = $$\frac{dis\tan ce}{time\ }$$