

Speed of Motorboat in still water= 18km/hr
Speed of stream= s km/hr
Effective Speed of motorboat when it goes upstream= 18-s
Effective speed of motorboat when is goes downstream= 18+s
Since time taken equals distance travelled divided by speed,
Time taken to tavel 24km upstream= 24/(18-s)
Time taken to travel 24km downstream= 24/(18+s)
According to the question, boat takes one hour more when it travels 24km upstream compared to downstream, so
{24/(18-s)} - {24/(18+s)} = 1
Solving this equation, you get s= 6 km/hr
