1 x 1 triangles: 3
1 x 2 triangles: 2
1 x 3 triangles: 1
2 x 1 triangles: 3
2 x 2 triangles: 2
2 x 3 triangles: 1
3 x 1 triangles: 3
3 x 2 triangles: 2
3 x 3 triangles: 1
4 x 1 triangles: 3
4 x 2 triangles: 2
4 x 3 triangles: 1
Now, add them up...