Explanation:
p + 40^o = 100^o (exterior ∠= sum of two interior opp ∠s)
p = 100^o - 40^o
P = 60^o
But q + p = 180^o (∠s on a straight line)
q + 60^o
q = 180^o - 60^o
q = 120^o
x = q (corresponding ∠)
∴x = 120^o