Answer:
40°
Step-by-step explanation:
[tex] m\angle A + m\angle B = 180\degree [/tex]
(Linear pair angles)
[tex] 140\degree + m\angle B = 180\degree [/tex]
[tex] m\angle B = 180\degree -140\degree [/tex]
[tex] m\angle B = 40\degree [/tex]
