Math Is Fun Forum

I try posting about this topic before but I got confuse with the site sorry

I would be grateful if someone would explain how to answer the following questions.
Thank you in advance!

[picture added bobbym]

12. If ABC is a 30-60-90 triangle, with angle ACB at 30 degrees, and line segment AC is the diameter of the circle, then if the length of line segment AB is 4, what is the radius of the circle?

A8
B2
C10
D4
E 7
F 16

13. Working with the information from 12 from here to #16, what is the measure of arc AB?

A90 deg
B25 deg
C30 deg
D120 deg
E 60 deg
F 72 deg

14. What is the measure of arc BC?

A20 deg
B45 deg
C75 deg
D120 deg
E 15 deg
F 90 deg

19. If I created an imaginary point on arc AC named D, and drew line segments from points A and C to D, what would the measure of angle ADC be?

A60 deg
B7.5 deg
C30 deg
D15 deg
E 90 deg
F 120 deg

20. If line segment BC has a length of 24, and line segment AB has a length of 18, what is the radius of the circle?

A18
B38
C6
D15
E 4
F 29