ANSWER
[tex]40feet[/tex]EXPLANATION
Given;
[tex]\begin{gathered} Area=1600ft^2 \\ width=40ft \end{gathered}[/tex]Recall that the area is calculated using;
[tex]area(ft^2)=lenght(ft)\times width(ft)[/tex]Substitute the values;
[tex]\begin{gathered} area(ft^2)=lenght(ft)w\imaginaryI dth(ft) \\ 1600=l\times40 \\ l=\frac{1600}{40} \\ =40 \end{gathered}[/tex]