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The measures of the angles in a triangle add up to 180°.
m \angle A + m \angle B + m \angle C= 180^\circ \\&#10;30^\circ + m \angle B + 20^\circ = 180^\circ \\&#10;m \angle B + 50^\circ = 180^\circ \\&#10;m \angle B = 180^\circ - 50^\circ \\&#10;m \angle B= 130^\circ

The measure of B is 130°.