The sum of the internal angles in a quadrilateral is equal to 360, therefore:
[tex]\begin{gathered} \angle A+\angle B+\angle C+\angle D=360 \\ 90+\angle B+50+\angle B=360 \\ 140+2\angle B=360 \\ 2\angle B=360-140 \\ 2\angle B=220 \\ \angle B=\frac{220}{2} \\ \angle B=110 \end{gathered}[/tex]