Given that,

AB ‖ CD and transversal cuts them

Let,

∠1 = 120^o + x and

∠2 = x

∠1 = ∠3 (Alternate angles)

∠3 = 120^o + x (i)

∠2 + ∠3 = 180^o (Linear pair)

x + 120^o + x = 180^o

2x = 60^o

x = 30^o