The number of straight lines that can be formed by joining $8$ points on a circle is:

In how many ways can $5$ boys and $5$ girls stand in a row so that no two girls may be together?

If $p$ points are taken on each of the three coplanar parallel lines,what is the maximum number of triangles that can be formed with vertices at these points?

The number of diagonals in a polygon is $20$. The number of sides of the polygon is:

Six '$X$'s have to be placed in the squares of the figure such that each row contains at least one '$X$'. In how many different ways can this be done?

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