55 degrees
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Explanation:
Let B represent the unmarked angle on the left between angle A and the 145 degree angle.
Angle B and the 145 degree angle form a straight line of 180 degrees
B+145 = 180
B = 180-145
B = 35
Then use the fact that angles A and B form a 90 degree angle
A+B = 90
A+35 = 90
A = 90-35
A = 55