In Dhanmondi Lake the current flows at 3 kilometres per hour. The boatmen takes twice as long to travel upstream between two points as it does to travel downstream between the same two points. In still water what is the speed of the boat?
A
3 miles per hour
B
6 miles per hour
C
9 miles per hour
D
12 miles per hour
Correct Answer: 9 miles per hour
Let the boat speed and current flow be U and V respectively. According to the question, the relative speed of boat downstream (U + V) is twice as that of upstream (U - V). So, U + V = 2(U - V) or, U + V = 2U - 2V or, 2U - U = V + 2V or, U = 3V = 3 x 3 = 9 km per hour