In a triangle the interior angles must add up to 180, so to find angle 1 you must subtract angles 2 and 3 from 180:
180-100 = 80
80-50 = 30
So the measure of angle 1 is 30. Now because we know that a straight line is 180 degrees, we know that if there is a line connected to it, the angles that it forms must add back up to 180. So we would subtract angle 1 from 180 to get angle 4:
150 is the correct answer. The sum of all angles in a triangle is 180. If Angles 2&3 add up to 150, angle 1 has to be 30. Angle 1&4 make up a supplementary angle, which is always 180 degrees.