Answer
$l \lt 400$
Work Step by Step
The longest natural arch is $400$ ft. All other natural arches will only have a length less than $400$ ft. Hence, the equation that denotes the length of all natural arches will be less than $400$ $$l \lt 400.$$ In the graph, since $400$ is not included, a hollow mark is made. The numbers less than $400$ is marked with a blue line.
