Multiple choice

A person can row 5 m/sec in still water. The speed of the stream is 6 km/h. The time (in hours) taken by him to row a distance of 76.8 km downstream is:

  1. $3.5$
  2. $2.8$
  3. $3.2$
  4. $2.4$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Speed in still water = 5 m/sec. Speed of stream = 6 km/h = 6 × (1000/3600) = 5/3 m/sec. Downstream speed = (5 + 5/3) = 20/3 m/sec = (20/3) × (18/5) = 12 km/h. Distance = 76.8 km. Time = Distance/Speed = 76.8/12 = 6.4 hours. Wait, this doesn't match the options. Let me recalculate: Stream speed = 6 km/h = 6000/3600 = 5/3 m/sec. Downstream speed = 5 + 5/3 = 20/3 m/sec. In km/h: (20/3) × 3.6 = 24 km/h. Time = 76.8/24 = 3.2 hours. This matches option C.

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