Question

The number of triangles formed by the 11 points, out of which 5 are collinear in a plane is

• A. 165
• B. 155
• C. 150
• D. None of theses

Answer 1

Answer: (C)

150

Answer 2

Number of triangles formed by 11 points $=$ 11C3
$=\frac{11!}{3!\times8}=165$
Number of triangles formed by 5 points (which are colinear) $=$ 5C3
$=\frac{5!}{3!\times2!}=10$
Number of triangles $=165-10=155$
The correct option is (C)