Correct Answer - Option 1 : 5(5/8)
Given:-
Boat speed = 9 km/hr
Formula:-
Speed = Distance/Time
Upstream speed = u - v
Concept:-
Boat will come back only if it is travelling upstream as after some time stream speed will be more than boat speed and boat will start travelling in backwards direction.
Calculation:-
Distance travelled in first hour = (9 - 2) × 1 = 7 km
Distance travelled in second hour = (9 - 2 - 3) × 1 = 4 km
Distance travelled in third hour = (9 - 2 - 3 - 3) × 1 = 1 km
Distance travelled in fourth hour = (1 - 3) × 1 = - 2 km or 2 km in backward direction.
After 4 hours boat is at a distance of (7 + 4 + 1 - 2) = 10 km from starting point.
Distance in fifth hour = (- 2 - 3) × 1 = - 5 km
Now boat will be (10 - 5) = 5 km away
Time for next 5 km = 5/(- 5 - 3) = 5/8 hour (ignore negative sign for time)
So boat will be back in 5(5/8) hours.