Ab and bc are two tangents to a circle which intersect outside the circle at a point b. if the measure of arc ac is 160°, what is the measure of angle abc?

The angle formed by the intersection of 2 tangents outside the circle equals half the difference of the intercepted arcs.

arc AC = 160° (given)
arc ADC = 360 - 160 = 200°

∠ABC = (200-160)/2= 20°  ← answer