Q. A man car row 15 km/hr downstream and 9 km/hr upstream, the speed of the boat in still water is:
Answer: 12 km/hr
Notes: Speed of boat in still water = $ = \frac{1}{2}(Rate downstream + Rate upstream) = \frac{1}{2}(15+9)$ $ = \frac{1}{2}\times 24 = 12 km/hr$ Hence option [C] is correct answer.