A man rows from J to K (upstream) and back from K to J (downstream) in a total time of 15 hours. The distance between J and K is 300 km. The time taken by the man to row 9 km downstream is identical to the time taken by him to row 3 km upstream. What is the approximate speed of the boat in still water?

Let's assume the speed of the boat in still water and the speed of the stream are 'B' and 'C' respectively.

A man rows from J to K (upstream) and back from K to J (downstream) in a total time of 15 hours. The distance between J and K is 300 km.

$$\frac{300}{B+C}+\frac{300}{B-C}\ =\ 15$$

$$\frac{60}{B+C}+\frac{60}{B-C}\ =\ 3$$    Eq.(i)

The time taken by the man to row 9 km downstream is identical to the time taken by him to row 3 km upstream.

$$\frac{9}{B+C}=\frac{3}{B-C}$$

$$\frac{3}{B+C}=\frac{1}{B-C}$$

3B-3C = B+C

3B-B = C+3C

2B = 4C

B = 2C

C = 0.5B

Put the value of 'C' in Eq.(i).

$$\frac{60}{B+0.5B}+\frac{60}{B-0.5B}\ =\ 3$$

$$\frac{60}{1.5B}+\frac{60}{0.5B}\ =\ 3$$

$$\frac{40}{B}+\frac{120}{B}\ =\ 3$$

$$\frac{160}{B}\ =\ 3$$

$$\frac{160}{3}\ =\ B$$

Approximate speed of the boat in still water = B = 53.33 km/h