The sum of all angles in a triangle = 180°, so: y°+y°+O=180°,==> 2y°+O=180°
Let's calculate O (angle NOP). We notice that O is the supplement of 120°
That mean O = 180°-120° = 60°. Now let's go back to 2y°+O=180
2y° + 60° = 180° ==> 2y° = 120° & y° = 60°
