In one hour, a boat goes 7 km/hr along the stream and 5 km/hr against the stream. The speed of the boat in still water (in km/hr) is: Choose the correct choice from the following choices:
A) 3 km/hr	B) 4 km/hr	C) 5 km/hr	D) 6 km/hr	E) 8 km/hr
Upstream relative speed is u + v=7km/hr   Downstream speed is u-v = 5   Where u = speed of boat in still water and v is speed of stream   Then adding two equations u+v + u-v =7+5   2u=12   Finally, u=6.   ANSWER : 6 km/hr
<result>D</result>
In one hour, a boat goes 7 km/hr along the stream and 5 km/hr against the stream. The speed of the boat in still water (in km/hr) is: