Each large triangle can be divided into 4 identical smaller triangles.
Since there are 4 large triangles in the square, total number of identical smaller triangles = 4 x 4 = 16
Fraction that is shaded = 1/16

Or
Shaded part is ¼ of the large triangle.
But large triangle is ¼ of the square.
¼ of large triangle = ¼ of ¼ of square
= ¼ x ¼ of square
= 1/16 of square
So fraction of the square that is shaded
= 1/16

Percentage = 1/16 x 100% = 6.25%
Or
100% → 16 units
1 unit → 100% ÷ 16 = 6.25%