A ship is travelling due east at a speed of 15 km/h. Find the speed of a boat heading 30° east of north, if it always appears due north from the ship.

where, v_bg is the velocity of boat w.r.t. ground and V_sg is the velocity of ship w.r.t. ground.
Relative velocity of boat w.r. t. ship v_bs is along north.
v_bs is resultant of v_bg and v_sg in opposite direction

                         $\mathrm{v} \cos {30}^{\mathrm{o}} = {\mathrm{v}}_{\mathrm{bs}}$ 

                          $\mathrm{v} \cos {30}^{\mathrm{o}} = 15 \Rightarrow \mathrm{v} = 30 \mathrm{km}/\mathrm{h}$