【Solution】Solution: $1 \div 3=\frac{1}{3}$
$$
\begin{array}{l}
2 \div 4=\frac{1}{2} \\
3 \div 5=\frac{3}{5} \\
\frac{1}{3}: \frac{1}{2}: \frac{3}{5}=10: 15: 18
\end{array}
$$
1 hour 26 minutes $=86$ minutes
$$
\begin{array}{l}
86 \times \frac{18}{10+15+18} \\
=86 \times \frac{18}{43} \\
=36 \text { (minutes) } \\
=0.6 \text { (hours); }
\end{array}
$$

Answer: The lamb took 0.6 hours to travel the downhill road. Therefore, the answer is: 0.6.
20. (6 points) A road consists of three sections: uphill, flat, and downhill, with the ratio of their distances being 1:2:3. The ratio of the speeds at which the little sheep passes through each section is 3:4:5, as shown in the figure. It is known that the little sheep took a total of 1 hour and 26 minutes to pass through the three sections. Then, the little sheep took $\qquad$ hours to pass through the downhill section.