Answer: 100
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Step-by-step explanation:
Let's add angle c to the diagram such that it's adjacent to angle b, and inside the quadrilateral. Notice how angle c is opposite the 100 degree angle of this inscribed quadrilateral.
For any inscribed quadrilateral, the opposite angles are supplementary
c+100 = 180
c = 180-100
c = 80
Angles b and c are supplementary as well, because they form a straight line.
b+c = 180
b+80 = 180
b = 180-80
b = 100
In short, angle b is the same measure as that 100 degree angle in the diagram.