Answer: 30 degrees, 60 degrees and 90 degrees
If one exterior angle is 120, then its interior counterpart is 180-120 = 60.
If another exterior angle is 150, then is interior counterpart is 180-150 = 30
Let x be the missing interior angle. It adds with the other interior angles and all three interior angles of any triangle always add to 180
x+60+30 = 180
x+90 = 180
x+90-90 = 180-90 .... subtract 90 from both sides
x = 90
So the missing interior angle is 90 degrees
