Speed of stream is 4 km/hr and the speed of boat is 11 km/hr. In how much time will the  boat cover a distance of 21 km upstream and 45 downstream ?

6 hours 

We know that,

Upstream Speed = Speed of boat - Speed of stream

Downstream Speed = Speed of boat + Speed of stream

We have,

Speed of stream = 4 km/hr.

Speed of boat = 11 km/hr.

Upstream Speed = 11 - 4 = 7 km/h

Upstream distance = 21 km

Time  = 21/7 = 3 hrs

Downstream Speed = 11 + 4 = 15 km/h

Downstream distance = 45 km

Time = 45/15 = 3 hrs

Total time = 3 + 3 = 6 hrs