If the perimeter of a circle with a 1/4 section taken out is 20cm, how do you calculate the radius?

You should know that the length of the perimeter of a circle is P = 2\pi R , where R is the radius of the circle, so the perimeter of 3/4 of a circle is 3/4 of that amount. The two edges of the 1/4 cutout are each length R. Now - can you set up an equation that sets the total perimeter (20 cm) equal to the sum of these parts? Post back and let us know what you find for R.