A boat takes one-third the time travelling the same distance when it goes downstream than upstream. What is the ratio of the speed in still water to the speed of the current?
1). 3 : 1
2). 2 : 1
3). 1 : 2
4). 1 : 3
Let the distance travelled be x km
Let the speed of current be v km/hr
Let the speed of boat in still water be b km/hr
∴ Speed of boat upstream = (b – v) km/hr
⇒ Speed of boat downstream = (b + v) km/hr
∴ Time taken to travel x distance upstream = x/(b – v) hr
⇒ Time taken to travel x distance downstream = x/(b + v) hr
Now, $(\frac{{{\rm{x}}/\left( {{\rm{b\;}}-{\rm{\;v}}} \right){\rm{\;}}}}{{{\rm{x}}/\left( {{\rm{b\;}} + {\rm{\;v}}} \right){\rm{\;}}}})$ = 3
⇒ b + v = 3(b – v)
⇒ b + v = 3b – 3v
⇒ 4v = 2b
⇒ b/v = 2 : 1
∴ Ratio of speed in still water to speed of current is 2 : 1