The speed of a boat in still water is 18 km/h and the speed of the stream is 3 km/h. How much time (in hours) will it take to cover a distance of 105 km downstream and come back?
Show answer & explanation
Correct answer: A
Explanation
The correct answer is Option 1 i.e. 12.
We know that,
Downstream speed = x + y and,
Upstream speed = x - y
here, x = speed of water and y = speed of stream
So,
Downstream speed = 18 + 3 = 21 km/hr
Time taken in downstream = 105/21 = 5 hrs
Now,
Upstream speed = 18 - 3 = 15 km/hr
Time taken in upstream = 105/15 = 7 hrs
Thus,
Total time = 5 + 7 = 12 hrs
Hence,
The total time to cover 105 km downstream and return upstream is 12 hours