there are 360 degress in a circle if you cut a circle in half at its center aka its diameter the center angle would be 180 degree or a straight line 360/2 180 and a circle divide into 2 sectors so your questoin would be equal algebraically to 360x=30 360 is the totatal number degrees in every circle 30 is the resulting degree and x is the length of each of the sectors if you solve yo get 1/12 thus total sectors would be 12