Answer
$B = 35.26^\circ$
Work Step by Step
$\sin{B} = \dfrac{b \sin{A}}{a} = \dfrac{20 \times \sin{120^\circ}}{30} $
$\sin{B} = \dfrac{\sqrt{3}}{3}$
$B = 35.26^\circ$
$B' = 180-B = 144.74^\circ$
$A+B' = 120+144.74 = 264.74 > 180 \hspace{20pt} \therefore B'$ is refused.
