Answer:
A quadrilateral's interior angles have a sum of 360°.
Given that we have 85°, 100°, and 90°(the right angle), and an unknown angle.
We can add these angles plus the unknown angle to equal a total of 360°.
85 + 100 + 90 + x(our unknown missing angle) = 360
simplify
275 + x = 360
Isolate x by subtract 275 from both sides;
-275 -275
x = 85, ∠1 = 85°
We can also double check:-
85 + 100 + 90 + 85 = 360
360 = 360 which is correct.