mohon bantuannya no 20

sudut CBA = 180-110 = 70
sudut CAB = 180-70-70 = 40

<B=110° ,<ACB=70°
<B+<ABC=180°
110+<ABC=180
<ABC=180–110
<ABC=70°

ΔABC=180°
<BAC+<ACB+<ABC=180
<BAC+70+70=180
<BAC+140=180
<BAC=180–140
<BAC=40°
semoga membantu ya...