Blackjack (21)
For Time
20 Push-Ups, 1 Sit-Up
19 Push-Ups, 2 Sit-Ups
18 Push-Ups, 3 Sit-Ups
...continue this pattern until...
2 Push-Ups, 19 Sit-Ups
1 Push-Up, 20 Sit-Ups

for the curious it ends up being 210 reps for each