How many triangles can be formed in a decagon?

Total number of sides in a decagon = 10

We know that number of vertices in triangle $=3$

So, out of 10 vertices we have to choose 3 vertices.

Therefore,

Total number of triangles in a decagon $={ }^{10} \mathrm{C}_{3}$ $=\frac{10 !}{3 ! \times(10-3) !}=\frac{10 \times 9 \times 9}{3 \times 2}$

Total number of triangles = 120