Answer:
2°
Step-by-step explanation:
Since, angles A and B are complementary.
Therefore,
[tex] m\angle A + m\angle B = 90\degree \\\\
(4x +40)\degree + (x-10)\degree = 90\degree \\\\
(4x+x +40-10)\degree = 90\degree \\\\
(5x +30)\degree = 90\degree \\\\
5x + 30= 90\\\\
5x = 90 - 30\\\\
5x = 60\\\\
x = \frac{60}{5}\\\\
x = 12\\\\
m\angle B = (x-10)\degree \\\\
m\angle B = (112-10)\degree \\\\
m\angle B = 2\degree \\\\[/tex]