Explanation:

- The man rows from A to B and from B to A in a total of 12 hours. The distance is 240 km each way.

- For 6 km downstream, the time taken is the same as the time for 4 km upstream.

- Let row speed in still water be \( x \) km/h and stream speed be \( y \) km/h.

- Downstream speed: \( x + y \) and upstream speed: \( x - y \).

- Time for 6 km downstream = Time for 4 km upstream implies:

$$

\frac{6}{x+y} = \frac{4}{x-y}

$$

- Solving: \( 6(x - y) = 4(x + y) \) gives \( 2x = 10y \) or \( x = 5y \).

- Total time is 12 hours:

$$

\frac{240}{x-y} + \frac{240}{x+y} = 12

$$

- Substitute \( x = 5y \) in the above equation:

$$

\frac{240}{5y - y} + \frac{240}{5y + y} = 12

$$

- Simplifies to:

$$

\frac{240}{4y} + \frac{240}{6y} = 12 \to \frac{60}{y} + \frac{40}{y} = 12 \to \frac{100}{y} = 12

$$

- Solving for \( y \): \( y = \frac{100}{12} = \frac{25}{3} \) km/h.

- The correct speed of the stream is 25/3 km/h.