A boat with a speed of 5 km/hr in still water crosses a river of width one km along the shortes possible path in 15 minutes. Find the velocity of river current in km/hr.

In still water, 
Distance=1km
Speed , s=5km/hr
So, Time taken = 1/5 hr

In the flowing river, 
Distance = 1km as the boat is supposed to take the shortest route (directly opposite)
Time = 15 min = 15/60hr =1/4 hr
Resultant speed of boat in river , u = 1/(1/4) = 4 km/hr

Let the speed of river be v
Using pythagoras theorem in the velocity triangle 
u^2=s^2-v^2
4^2=5^2-v^2
v=3km/hr