When you have parallel lines same side interior angles add up to 180 degrees. Let the first SSI (same side interior) angle be x, and the other SSI be (x - 20). Therefore, x + (x - 20) = 180 2x - 20 = 180 add 20 to both side 2x = 200 divide both sides by 2 x = 100 x - 20 = 100 - 20 = 80
