For 20 minutes, perform:
Strict pull ups - push-ups - squats
5-10-15 reps
If you can finish 8 rounds by 5:00, switch to
10-20-30 reps
If you can finish 4 rounds by 10:00, switch to
20-40-60 reps
If you can finish 2 rounds by 15;00, switch to
40-80-120 reps, then stop.