I think it is important for your child to memorise the properties of angles related to parallel lines.
With reference to the diagram above,
∠a = ∠e ; ∠b = ∠f ; ∠d = ∠h ; ∠c = ∠g
∠d = ∠e ; ∠c = ∠ f
Sum of angles between parallel lines = 180º
∠c + ∠e = 180º
∠d + ∠f = 180º