My Negative Contrast Area

The length of side of the square is $\sqrt{225}=15$ . Observed that the length $CD$ is the sum of the lengths of the horizontal sides of the black rectangles and the length $BC$ is the sum of the lengths of the vertical sides of the black rectangles. That's why the perimeter of the black region is $15+15+15+15=$ $\color{#D61F06}\boxed{\large 60}$

$\sqrt{225}=15$ so the square has side length $15$

Consider the vertical perimeter of the region. On one side we have the side of the square that has length $15$ . The vertical side length on the other side is just the side of the square split up so also has length $15$ .

We have a similar result for the horizontal perimeter so our overall perimeter is:

$15 \times 4=\boxed{60}$

Move the black edges to the side. Then it is obvious that the perimeter is 60 since sqrt(225)=15 15*4=60