If AB||CD then let’s write the measurement of the angles given below:

Given that AB||CD and EF is the transversal.

So, if two lines are parallel and cut by a transversal then the corresponding angles are equal,

Alternate interior angles are equal

Alternate exterior angles are equal,

Sum of co-interior angles is 180°.

∠4=100° , ∠7=x

∠6=∠7=x (vertically opposite angles)……………(1)

Now, as AB||CD,

∠6+∠4 =180°

⇒ x+100° =180° (co-interior angles)

⇒ x=180°-100°

⇒ x=80° (from (1))

Hence, x=80°