One possible way: draw 21 radii, each forming an angle of 17.142857.. degrees with another radius.
Another option: the question does not require the 21 parts to be equal. So make twenty cuts where each cut divides a segment into two (equal or unequal) parts.
Not everyone knows ?
You just divide the circles into 4 parts
Make a horizontal line and vertical line through the circle. You now have 4 parts. Make a line from the center to the edge of the circle in 3 of these parts and you now have 7 parts.
A circle divides a plane into three parts.
draw a + inside it, touching the outside of the circle-cut it into Quaters.
Cut/draw 21 lines from the center out.
21 divide by 16 = 1.3125 inches
The three parts are:the interior of the circle, the points on the circle (points on its circumference) the outside of the circle.
Only the diameter can divide the circle in to two equal parts.
Go to a pizzeria and get them to do it for you.
Construct a circle with a 4.5 radius. The circle's circumference is 360 degrees. So mark out 3 by 120 degrees on the circumference and join them to the centre of the circle which will divide the circle into three equal parts.
Not everyone knows ?
You just divide the circles into 4 parts
Make a horizontal line and vertical line through the circle. You now have 4 parts. Make a line from the center to the edge of the circle in 3 of these parts and you now have 7 parts.
A circle divides a plane into three parts.
A circle divided into 12 parts will have 12 arcs of 30 degrees around its circumference.
Eight of them.