Answer: 28 shaded triangles
Explanation:
First, we have 0 shaded triangles.
Then we have 1 shaded triangle
then we have 3 shaded triangles
then we have 6 shaded triangles
In each new triangle, we add a base with a larger number of triangles.
In the base of the fourth triangle we have 3 triangles, then in the next triangle we have a base of 4 triangles, then we have 4 + 3 + 2 +1 triangles = 10 triangles.
in the sixth triangle the base has 5, so here we have 5 + 4+ 3 + 2 +1 = 15 triangles
in the seventh the base has 6 triangles, so here we have 15 + 6 = 21 shaded triangles
and in the height triangle, the base has 7 triangles, then we have 28 shaded triangles