Each point on the perimeter is equidistant from the centre. and because it is. what type of question is that anyways?
A quarter of a circle is 2/8 of a circle. Add to that an eighth (1/8) of a circle (not eight) and this equals 3/8 of a circle. This equates to 37.5 % of a circle.
arc
Half.
Concentric Circles?