Answer
$1\ ft$
Work Step by Step
Step 1. Assume the width of the border is $x$, the size of the center rectangle would be $9-2x$ by $5-2x$
Step 2. The area of the center rectangle would be $A_1=(9-2x)(5-2x)$
Step 3. The total area is $A=9\times5=45$ and the area of the border would be $A_2=A-A_1=45-(9-2x)(5-2x)=18x+10x-4x^2=28x-4x^2$
Step 4. Let $A_2=24$, we have $28x-4x^2=24$ or $x^2-7x+6=0$ which gives $x=1\ or\ 6$
Step 5. Discard $x=6$ as it is not practical. Thus $x=1\ ft$