Let the width of one rectangle be 1 unit.

Base of the shaded triangle
= 2 x width of a rectangle
= 2 units

Height of shaded triangle is the same as the length of the rectangles.

Area of shaded triangle
= ½ x base x height of shaded triangle
= ½ x 2 units x height of shaded triangle
= 1 unit x height of shaded triangle

Area of the whole figure (the big rectangle)
= width x length
= 4 units x length
= 4 units x height of shaded triangle

From above, you can see that the shaded triangle is ¼ of the whole figure

Another way to solve this:

You should be able to see one unshaded rectangle and two unshaded triangles.

One of the unshaded triangles is ½ of 3 rectangles.
So its area = ½ x area of 3 rectangles
= area of 1½ rectangles.

The other shaded triangle is half of 1 rectangle.
So its area = ½ x area of 1 rectangle
= area of ½ rectangle.

Total unshaded area
= Area of 1½ rectangles + area of 1 rectangle + area of ½ rectangle
= Area of 3 rectangles

Area of whole figure = area of 4 rectangles

So, unshaded area = area of 4 rectangles
- area of 3 rectangles
= area of 1 rectangle

So you get the same result, ¼ of the figure is shaded.