Quadrilateral ABCD is inscribed in this circle.

What is the measure of angle B?

Opposite angles in a cyclic quadrilateral are supplementary....so....

x + 4x - 20 = 180 add 20 to each side

5x = 200 divide both sides by 5

x = 40°

So....angle B = (4(40) - 20) = 160 - 20 = 140°