A boat goes 60 km downstream in 6 hours to reach the destination.The boat will take total 21 hour if it has to go back to the starting point and come back to the destination. Find the speed of boat in still water? (in km/hr) गंतव्य तक पहुंचने के लिए 6 घंटे में एक नाव 60 किमी धारा के साथ जाती है, और वापस आ जाती है यदि कुल 21 घंटे लगे हों । तो शांत जल में नाव की चाल क्या होगी? (किमी / घंटा में)

Wait, the question says downstream takes 6 hours for 60 km, so downstream speed = 60/6 = 10 km/hr. Then it says 'total 21 hour if it has to go back to the starting point and come back to the destination'. This means: downstream + upstream = 21 hours. So 6 + upstream_time = 21, giving upstream_time = 15 hours for 60 km. Upstream speed = 60/15 = 4 km/hr. Boat speed in still water = (downstream + upstream)/2 = (10 + 4)/2 = 7 km/hr. This matches option C (7).