18. (3 points) Li Shuang rides a bike at a speed of 320 meters per minute from location $A$ to location $B$. On the way, due to a bicycle malfunction, he pushes the bike and walks for 5 minutes to a place 1800 meters from $B$ to repair the bike. After 15 minutes, he continues towards $B$ at 1.5 times his original riding speed, and arrives at $B$ 17 minutes later than the expected time. What is Li Shuang's walking speed in meters per minute?
【Solution】Solve: $1800 \div 320-1800 \div(320 \times 1.5)$
$$
\begin{array}{l}
=5.625-3.75 \\
=1.875 \text { (minutes) } \\
320 \times[5-(17-15+1.875)] \div 5 \\
=320 \times[5-3.875] \div 5 \\
=320 \times 1.125 \div 5 \\
=360 \div 5 \\
=72 \text { (meters/minute) }
\end{array}
$$

Answer: Li Shuang's speed of pushing the cart while walking is 72 meters/minute.
Therefore, the answer is: 72.