Correct Answer - Option 2 : 10 hours
Given:
The speed of a ship in still water = 5 km/hr.
The speed of the stream = 2 km/hr.
Total distance = 21 km
Formula used:
Speed = Distance/ time
Calculation:
Relative speed of downstream = speed of a ship in still water + speed of the stream
⇒ Relative speed of downstream = 5 + 2
⇒ 7 km/hr
Relative speed of upstream = speed of a ship in still water – speed of the stream
⇒ Relative speed of upstream = 5 – 2
⇒ 3 km/hr
Time taken when go with upstream = 21/3 = 7 km/hr
Time taken when go with downstream = 21/7 = 3 km/hr
Total time = 3 + 7 = 10 km/ hr
∴ Total time taken is 10km/hr.
