Every minute on the minute perform 5 pull-ups, 10 push-ups and 15 squats
Can you continue for 30 minutes? 20 minutes? How about 10? If you fall behind, keep going for 30 minutes and see how many rounds you can complete.
If you've finished the workout before, add +1 to each exercise, i.e., 6 pull-ups, 11 push-ups, 16 squats every minute on the minute. See if you can go the full 30 minutes. We've seen +3. Any +4s?