#13: A set of 9 parallel lines intersects with another set of 10 parallel lines. How many parallelograms can be formed in total?
#14: (a) 8 points in the space, any 4 points are not coplanar (in the same plane). If every 3 points are used to make a plane, how many planes?
(b) 10 points in the space, any 4 points are not coplanar and each 4 points are used as vertices to make a tetrahedron, how many tetrahedron?
#35: There are 9 workers, of which 4 are fitters, 3 are turners, and the other 2 are both fitters and turners. It is planned to select 2 fitters and 2 turners from these 9 workers to complete a task. How many ways?