If $t_n$ denotes the number of triangles formed with $n$ points in a plane,no three of which are collinear,and if $t_{n+1}-t_n=36$,then $n$ is equal to

There are $n$ distinct points on the circumference of a circle. If the number of pentagons that can be formed using these points as vertices is equal to the number of triangles that can be formed,then what is the value of $n$?

How many triangles can be formed using $5$ points on a line and $3$ points on a parallel line?

How many parallelograms can be formed by a set of $4$ parallel lines intersecting another set of $3$ parallel lines?

$l_1, l_2, l_3$ are three parallel lines in the same plane. If there are $m$ points on $l_1$,$n$ points on $l_2$,and $k$ points on $l_3$,what is the maximum number of triangles that can be formed with vertices at these points?

The greatest possible number of points of intersection of $8$ straight lines and $4$ circles is