I wonder what shape the window is? It has a radius so I expect a circle but you use the word perimeter which is usually reserved for shapes with sides.
So my guess is you are looking for the circumference of a circular window with radius 20. The circumference is given by (pi)d where d is the diameter. The diameter is twice the radius so 40. We use 3.14 to approximate pi.
That makes the circumference (the distance around the circle) = (40)(3.14)=125.6
