Correct Answer - Option 1 : 3 km/hr
Calculation
Time taken by dolphin to cover 14 Km along with stream and 5 Km against stream is 7 hours
⇒ Speed of dolphin in still water = SD = 4 Kmph
Let the speed of current be SW = x
⇒ 14/(4 + x) + 5/(4 - x) = 7
⇒ 7x2 - 9x - 36 =0
On solving the equation, we get,
⇒ x = 3, reject x = – 12/7
∴ Speed of current is 3 Kmph.
Speed of upstream = speed of boat in still water - speed of current
Speed of downstream = speed of boat in still water + speed of current