a quarter of a circle is 19.625
so the area of full circle is 19.625 x 4 = 78.5 sq. km
area of a circle is PI x r^2
78.5 = 3.14 x r^2
78.5/3.14 = 25 = r^2
r = sqrt(25) = 5
radius is 5 km
circumference ( perimeter) = 2 x PI x r
2 x 3.14 x 5 = 31.4 km
perimeter is 31.4 km