The formula to use is
z = (x+y)/2
where
x and y are the measures of the minor arcs (highlighted in blue) and z is the measure of the intersecting chord angles
In this case,
x = measure of minor arc DE = 30
y = measure of minor arc AC = unknown
z = measure of angle ABC = 90
So,
z = (x+y)/2
90 = (30+y)/2
2*90 = 30+y
180 = 30+y
y+30 = 180
y = 180-30
y = 150
Answer: C) 150 degrees
