After declaration of Sadiya Dhubri stretch in Brahmaputra river as a National Waterway Assam Tourism...........

After declaration of Sadiya-Dhubri stretch in Brahmaputra river as a National Waterway, Assam Tourism Department decides to do a survey of landscape surrounding the stretch of the river. A boat of Assam Tourism Department has a speed of 5 km/hr in still water; crosses Brahmaputra river of width 30 km in Sadiya-Dhubri stretch along the shortest possible path in 15 hours. The speed of the river is?