Complete as many rounds as possible in 20 minutes of:

21 bench presses
21 L-pull-ups

The difficulty of each of these exercises should control the total volume of reps as muscular fatigue sets in over the course of 20 minutes. Each movement should be hard, but not so hard that you can only complete singles. If you make it too easy you’ll be flying through unbroken sets and end up performing an obscene number of reps by the end.