Let \( \Large v_{1} \) be the speed of boat in still water and \( \Large v_{2} \) be the speed of current Therefore, \( \Large v_{1} \) + \( \Large v_{2} \) = 20 ...(i) \( \Large v_{1} \) - \( \Large v_{2} = \frac{20}{2} \) = 10 ...(ii) From equations (i) and (ii) we get \( \Large v_{1} \) = 15 km/hr \( \Large t = \frac{d}{v_{1}} = \frac{30}{15} = 2 hrs. \)