b is an inscribed angle and an inscribed angle is half the measure of its intercepted are, and likewise, the intercepted arc is double the angle, so b = 90. a is a central angle which has the same measure as its intercepted arc. So a = 90. the measure of c is half of its intercepted arc, which is 140, so c = 70. The outside measure of a circle is 360, and we have the measure of 2/3 of it so we can figure out the measure of the arc that is intercepted by angle d. 360-140-90=130. Half of 130 is 65. a = 90, b = 90, c = 70, d = 65. A from above.
