Question

what is $m\angle 1$?
$m\angle 1 = \square ^\circ$

Explanation:

Step1: Find triangle's third angle

Sum of angles in triangle is $180^\circ$. Let the unknown interior angle be $x$.
$x = 180^\circ - 90^\circ - 30^\circ = 60^\circ$

Step2: Relate to $\angle 1$ (supplementary)

$\angle 1$ and $x$ form a straight angle ($180^\circ$).
$m\angle 1 = 180^\circ - 60^\circ$

Answer:

$120$