< 1 + < 2 = 180...straight lines = 180
< 3 + < 4 = 180...straight lines = 180
< 2 + < 3 + < 6 = 180....interior angles of a triangle = 180
so if < 1 and < 2 = 180, then < 2 = 180 - 105 = 75
and if < 3 and < 4 = 180, then < 3 = 180 - 115 = 65
now...< 2 + < 3 + < 6 = 180
75 + 65 + < 6 = 180
140 + < 6 = 180
< 6 = 180 - 140
< 6 = 40 <===
