A boat covers 36 km downstream in 4 hours and 18 km upstream in 6 hours. Find the speed of the boat in still water.

12 km/hr
9 km/hr
6 km/hr
5 km/hr

Correct answer: 6 km/hr

Solution

Downstream speed = 36/4 = 9 km/hr and upstream speed = 18/6 = 3 km/hr. The speed of the boat in still water is the average of downstream and upstream speeds, i.e., $(9+3)/2 = 6$ km/hr. So, the correct answer is 6 km/hr.

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