Three of the six vertices of a regular hexagon are chosen at random. The probability that the triangle with three vertices is equilateral is:

1/2 1/5 1/10 1/20

1/10

Total no. of triangles = ${^6C}_3$ Total no. of equilateral Δ's = 2 (ACE, BFD) Probability = $\frac{2}{20}=\frac{1}{10}$