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 vR+vb1=d   where vR and vb are the velocity of the river and man respectively/for upstream vb-vR3=d    solving we get vb=2vR  for crossing the river in shortest path vbrsinθ=vRsinθ=12 θ=300