Six, a boat sails between
$A, B$ two docks. When sailing downstream, it takes 40 minutes to be 4 kilometers away from the destination, and when sailing upstream, it needs $1 \frac{1}{3}$ hours to arrive. It is known that the upstream speed is 12 kilometers per hour. Find the speed of the boat in still water.
Six, let the speed of the current be $x$ kilometers/hour. Then
$$
\frac{2}{3} x+4=1 \frac{1}{3} \times 12 \text {. }
$$

Solving for $x$ gives $x=18$.
Therefore, the speed of the boat in the stream is
$$
(18+12) \div 2=15 \text { (kilometers/hour). }
$$

Answer: The speed of the boat in still water is 15 kilometers/hour.