18. A construction vehicle loads cement power poles from the warehouse and delivers them to the roadside 1000 meters away from the warehouse for installation. It is required to install a power pole every 100 meters along one side of the road. Given that the vehicle can carry a maximum of 4 poles per trip, and the task is to deliver 18 poles and return to the warehouse. If the vehicle consumes 汭车 liters of fuel per meter (here, the amount of fuel consumed is only related to the distance traveled, other factors are not considered), and each liter of gasoline costs $n$ dollars, what is the minimum fuel cost to complete the task?
18. Obviously, the fewer the number of trips, the shorter the distance traveled, and the less the fuel cost. Therefore, transporting 18 power poles in 5 trips results in a shorter travel distance. There are two methods for these 5 trips:
(1) Four trips of 4 poles each, and one trip of 2 poles;
(2) Three trips of 4 poles each, and two trips of 3 poles each.
(1) Consider sending 2 poles first, then 4 poles; or sending 4 poles first, then 2 poles.
(1) Sending 2 poles first, then 4 poles, the total distance for the two trips is:
$$
\begin{array}{l}
(1000+100) \times 2+(1100+400) \times 2 \\
=5200(\text { meters }) .
\end{array}
$$
(2) Sending 4 poles first, then 2 poles, the total distance for the two trips is:
$$
\begin{array}{l}
(1000+300) \times 2+(1300+200) \times 2 \\
=5600 \text { (meters). }
\end{array}
$$
(2) When sending 3 poles each time for two trips, the distance traveled is:
$$
\begin{array}{l}
(1000+200) \times 2+(1200+300) \times 2 \\
=5400 \text { (meters). }
\end{array}
$$

Therefore, sending 2 poles first results in the shortest travel distance. The shortest total distance is:
$$
\begin{array}{l}
(1000+100) \times 2+(1100+400) \times 2+(1500 \\
+400) \times 2+(1900+400) \times 2+(2300+400) \times 2 \\
=19000(\text { meters }) .
\end{array}
$$

The minimum fuel cost is
$$
19000 \cdot m \cdot n \div 1000=19 m n \text { (yuan). }
$$
(Provided by Nan Xiuquan, Hubei Huanggang City Educational Research Office)