besar sudut ABC adalah...

sudut c
= 180 - (3x - 20)
= 180 - 3x + 20
= 200 - 3x

sudut b
= 180 - (2x + 20)
= 180 - 2x - 20
= 160 - 2x

nilai x
70 + 200 - 3x + 160 - 2x = 180°
430 - 5x = 180
-5x = 180 - 430
-5x = -250
x = -250/-5
x = 50

sudut ABC
= 160 - 2x
= 160 - 2(50)
= 160 - 100
= 60°

<C = 70°
<B + <C + <A = 180°
{180° - ( 2x+20° )} + { 180° - ( 3X - 20° )} + 70° = 180°
160° - 2x + 200° - 3x = 180° - 70°
360° - 5x = 110°
360° - 110° = 5x
250° = 5x    ⇒⇒  x = 250/5 = 50°

Jadi sudut ( <ABC ) = 160° - 2 ( 50° ) = 160 - 100 = 60°