Correct Answer - Option 3 : 8 km
Given:
The speed of the boat in still water: The speed of river = 3 : 1
Time is taken for the round trip = 45 mins
Speed of the stream = 4 km/h
Formula used:
Speed = Distance/Time
Upstream = Speed of the boat - speed of the river
Downstream = Speed of the boat + speed of the river
Average speed = (2xy)/(x + y)
where x and y are speed
Calculation:
Time in hours
45/60 = 3/4 hours
The speed of the river is 4 km/h
Then the speed of the boat will be 3 × 4 = 12 km/h
Upstream = 12 - 4 = 8 km/h
Downstream = 12 + 4 = 16 km/h
Average speed = ( 2 × 8 × 16)/(8 + 16)
⇒ 256/24 = 32/3 km/h
Distance = (32/3) × (3/4) = 8 km
∴ The distance of the whole trip is 8 km.