Suzy,
So you have 5 triangles in the first triangle. Now look at the first two small triangles--together, they make one larger one, right? Now add the next one, the third small one, and you have another, still larger triangle.
Now look at the two small triangles in the middle--together, they make one mid-size triangle. Add to that the last of the small triangles and you have yet another larger one.
Now you have 10.