How would I find the perimeter of a regular pentagon inscribed in a circle which has a radius of 20 inches?

Make a sketch, draw two radii out to the vertices of the pentagon. Connect them. The central angle at the center of the circle is 360/5 =72. Draw the altitude from the center perpendicular to the side of the pentagon. You have a right triangle, with hypotenuse=20. The angle formed by the altitude and the radius is 36, half of the 72 degree central angle. Sin 36 = x/20. The perimeter will be 10x.