The image below shows a square inscribed inside a square. The vertices of the gray shaded square bisect the 4 sides of the larger square

To find the probabilities, the first step is to find the areas of the squares
First find the area of the larger square
The length is 2x
A = (2x)^2 = 4x^2
Then we need to find the area of the shaded square
That will be a 45 45 90 triangle so we can find the length of the hypotenuse,h. The hypotenuse is sqrt(2) times the length of the side of the triangle
h = x sqrt(2)
The side length for the shaded square is x sqrt(2) so
A = (x sqrt(2))^2 = 2 x^2
P( shaded area) = shaded area/ total area
= 2 x^2 / 4 x^2 = 1/2