Write the maximum number of points of intersection of 8 straight lines in a plane.

Write the value of .

There are 3 letters and 3 directed envelopes. Write the number of ways in which no letter is put in the correct envelope.

Write the number of parallelograms that can be formed from a set of four parallel lines intersecting another set of three parallel lines.

Write the number of ways in which 5 red and 4 white balls can be drawn from a bag containing 10 red and 8 white balls.