The exterior angle at the intersection of the tangent and secant has a measure that is half the difference between the intercepted arcs.
... ((10x+20) -80)/2 = 2x+15
... 5x -30 = 2x +15
... 3x = 45
... x = 15
So, the unknown arc to the right has measure
.. 10x + 20 = 10·15 +20 = 170
And the arcs of the circle total 360°.
... 80 + z + 170 = 360
... z = 360 - 250 = 110 . . . subtract 250 from both sides
The appropriate choice for the value of z is
... B. 110
