Answer:
25.
Explanation:
I am assuming you only mean the small triangles, not the composite ones.
The second triangle set has 3 triangles on bottom row and 1 on the top - total 4.
The 3rd triangle set has 5 triangles on the bottom , 3 on the next up and 1 on the top - total 9
Following the pattern 4th set will have the number of triaNGLES
7 + 5 + 3 + 1 = 16.
So the 5th will have 9 + 7 + 5 + 3 +1 = 25 triangles.