Call the side of the square , S
And its area = S^2
We can see that 3/4 of the area of the square is unshaded
So...we just need to find the area shaded in the remaining 1/4 th
Note that at the bottom right of the figure we have an isosceles right triangle with leg lengths of S / 2
So....the area of this triangle is (1/2) (product of the leg lengths) = (1/2) (S/2) (S/2) = S^2/8
But 1/2 of the area of this triangle plus the shaded area comprise the other 1/4th area of the square
So 1/2 of the area of the right triangle = (1/2)(S^2)/8 = S^2/16 = (1/16)S^2
So.....the shaded area = (1/4)S^2 - S^2/16 = (4/16)S^2 - (1/16)S^2 = (3/16)S^2
So....the shaded part is just 3/16 of the area of the square