Correct Answer: Option D
We can write three-quarters of a kilometer as 750 metres, and 11×(1/4) minutes as 675 seconds.
Rate upstream = (750/450) m/sec. = 10/9 m/sec.
Rate downstream = (750/450)m/sec = 5/3 m/sec
So, rate in still water = ½(10/9+5/3) m/sec
=25/18 m/sec
={(25/18)×(18/5)} km/hr
= 5 km/hr.
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