Depending on your objective – strength, size, fat loss – you can use many different strategies for a short workout. Let's say your goal is NOT fat loss. After all, it's easy to design a fat loss workout lasting 20 minutes or less. Let's instead look at strength.
When someone is strapped for time and his main goal is strength and size I recommend an EMOM approach. EMOM means that you start a set at the beginning of each minute (every minute on the minute). Here's an actual workout I did a few years back that completely destroyed me:
EMOM 21 minutes
- 4 front squats at 75%
- 4 bench presses at 75%
- 4 deadlifts at 75% (I actually did stone lifts, but most gyms don't have strongman stones)
You start a set at the beginning of each minute and the set (4 reps of one exercise) must be completed before the minute ends. If at one point you can't do the 4 reps unbroken, you can rest 10-15 seconds and finish it... it just gives you less rest before the next minute starts! Each minute you do a different exercise:
- Minute 1 = 4 front squats
- Minute 2 = 4 bench presses
- Minute 3 = 4 deadlifts
Repeat until the 21 minutes is up. Why 21? Because then each exercise has 7 sets.
If you prefer to focus more on the upper body you can replace the deadlifts with pull-ups. You could even do a power clean instead of the deadlift if you'd rather work on explosiveness (I would recommend 70% instead of 75% for cleans).
You WILL need a day off after this one!