16. Person A and Person B start from point A to point B at the same time. If both walk at a constant speed, A takes 4 hours to complete the journey, while B takes 6 hours to complete the journey. Then, when the remaining distance for B is 4 times the remaining distance for A, they have been traveling for $\qquad$ hours.
【Solution】Solution: The speed ratio of A and B is $6: 4=3: 2$; Considering the entire journey as 10 parts, A has walked 9 parts, then B needs to walk 6 parts;
$$
\begin{array}{l}
9 \times 4 \div 10, \\
=36 \div 10, \\
=3.6 \text { (hours). }
\end{array}
$$

Answer: They have been traveling for 3.6 hours.
Therefore, the answer is: 3.6.