If the radius of the circle is 10 inches, this means that its diameter is twice that, so its diameter is 20 inches.
The diameter can be draw goinf from the side AC to the side BD and it will make a 90° angle with these sides.
This means that the length of the diameter is the same as the length of the sides AB and CD and, since CABD is a square, all its sides have the same measure.
The perimeter is the sum of the measures of the sides of a the polygon, so, if the four sides measure 20 inches, the perimeter is:
The perimeter is 80in.