You would need to shade 1.5 boxes or 6 pieces to have shaded 25% of the shape.
In total, there are 6 boxes. There are 4 pieces per box. In total, there are 24 pieces.
6/4 = 1.5
24/4 = 6
You would need to shade 1.5 boxes, which is equal to 6 pieces or triangles.