A boat takes 11 hours to travel a distance upstream and takes 5 hours to travel the same distance downstream. If the speed of the boat in still water is 2 m/sec, then the speed of the stream (in km/h) is:
Show answer & explanation
Correct answer: B
Explanation
The correct answer is option 2 i.e. 3.
Let the speed of a stream be y km/h
The speed of the boat in still water = 2(2/9) = 20/9 m/s = (20/9) × 18/5 = 8 km/h
Hence,
Downstream speed = 8 + y
Upstream speed = 8 - y
Formula used:
Distance = downstream speed × time taken = upstream speed × time taken
Calculation:
(8 + y) × 5 = (8 - y) × 11
⇒ 40 + 5y = 88 - 11y
⇒ 16y = 88 - 40
⇒ 16y = 48
⇒ y = 48/16 = 3
Hence, speed of stream = 3 km/h