It's a K1 P1 rib. The next row is just continue in rib and slip markers. Later you do more decreases at the markers again, so it seemed like the decreases should be centered and keep the markers in place. I'm just new enough to feel like I shouldn't second guess the pattern on the 20th row, but it didn't make sense!! Not sure you can see the pattern but here's the link.
http://www.sweaterbabe.com/122-tie-f...led-long-vest/