Perimeter and Area of Common Plane Figures

A roller brush used for wall-painting has a circular cross section of diameter \(8\) cm, and the breadth of the roller is \(21\) cm. A painter uses this roller brush to paint a wall of breadth \(4.2\) m and height \(5\) m. He paints in vertical rectangular paint columns of breadth \(21\) cm and in a speed of \(10\) cm/s in the vertical direction.

Assuming there are no overlapping paint columns, find

1. the number of paint columns needed to finish painting the whole area of the wall,
2. the number of rotations of the roller to finish painting one column, correcting your answer to the nearest integer,
3. the least possible time needed for the painter to fully paint the wall.