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
