Correct Answer - Option 2 : 2.5 km/hr
Calculation:
Time for going upstream = 3 × Time for going downstream
The ratio of time for upstream to downstream = 3 ∶ 1
Since distance same, speed ∝ (1/time)
The ratio of speed for upstream to downstream = 1 ∶ 3
Speed of boat = (upstream speed + downstream speed)/2 = 4/2 units
Speed of boat = 2 units
2 units → 5 km/hr
Speed of stream = (downstream speed - upstream speed)/2 = 2/2 units
Speed of stream = 1 unit → 5/2 = 2.5 km/hr
∴ The speed of stream is 2.5 km/hr.
