Correct Answer - Option 2 : 6.75 km/hr
Concept Used:
Speed = Distance/Time
Calculation:
Suppose he rows 15 km downstream and 12 km upstream in x hours.
Speed(upstream) = 12/x km/hr and Speed(downstream) = 15/x km/hr
It is given that he can row to a place 60 km distant and back in 18 hours.
⇒ 60/(12/x) + 60/(15/x) = 18
⇒ 5x + 4x = 18
⇒ x = 2
Speed(upstream) = (12/x) km/hr = 12/2 = 6 km/hr
Speed(downstream) = (15/x) km/hr = 15/2 = 7.5 km/hr
∴ Speed of boat = ½(6 + 7.5) = 6.75 km/hr