A motor boat travels certain distance down stream in 1 hr and up stream in 3 hrs. Now the angle normal to the flow at which the same boat must be rowed with same velocity in the river in order to cross the river along the least path is          

if time taken in downstream is 1 hr for a distance d then $\left(v_R+v_b\right)1=d$where $v_R and v_b$ are the velocity of the river and man respectively/

for upstream $\left(v_b-v_R\right)3=d$    solving we get $v_b=2v_R$  for crossing the river in shortest path $v_{br}\sin \theta =v_R$

sin$\theta$=$\frac{1}{2}$ $\theta ={30}^0$