1. Start counting from the small triangles, which are 6 triangles.

2. Now consider combining two triangles. There are 3 large (2 combined) triangles, one per each side.

3. Now take into account, combining three triangles, there are 6 large (3 combined) triangles.

4. You may find it difficult to count 6, as you may end up with only 3. But there are 6.

5. Here is the simple clue, just count from each point on the large triangle and you will get 6 (3 combined) triangles, as there are 6 points on the large triangle.

6. Finally there is 1 outer large triangle.

7. Hence there are 6(single)+3(two combined)+6(three combined)+1(whole)=16 triangles.