He is tearing the angles of a triangle and arranging all three angles in such a way that there is no gap and there is no overlap. so, the resulting angle is the sum of all three angles. As we know, the sum of angles of a triangle is 180°. So, the resulting angle or sum of three angles of triangle is 180° or Straight angle.
All of the other answers share at least one factor they can be divided by (the numerator in all cases). 5/12 however shares no factors and cannot be simplified anymore.
