Three straight lines  l1,l2  and  l3 are parallel and lie in the same plane. Five points are taken on each of l1,l2 and  l3. The maximum number of triangles which can be obtained with vertices at these points, is

The total number of points is 15. From these 15 points we can obtain  15C3 triangles. However, if all the 3 points are chosen on the same straight line, we do not get a triangle. Therefore, the required number of triangles=15C3−3 5C3=425