Drag point B around the circle to see the relationship between x and the angle at B. Note how when x = 0, b = 0, and when x = 360, b = 90; that allows you to set up a simple proportion where x / 360 = B / 90; with the given information that the angle at B is 20, we can write x / 360 = 20 / 90, and x = 80.