20 minutes is a lot of time to stress your body out. actually, in terms of time, unless you have your own weight set in your aparmtment, weights are not a good option. you have to go to the gym, change, wait for the equipment, shower, change and leave. if you only have 20 minutes, do what senor saucemaster did and do burpees ad nauseum and you will soon be thinking that 20 minutes is too long for a workout. i guarantee you that you will not be able to do even 5 minutes of burpees non-stop without a break. what i'm trying to say is that if you only have 20 minutes, i would avoid the gym because preparations, etc. will take up most of your time. here's another suggestion, which only takes about 15 minutes. take a deck of cards and do the old catch wrestling workout. shuffle the deck. pull a card. if it's red, it's a squat, if it's black, it's a pushup. the number on the card determines how much you do. face cards are 10 reps. later you can increase this. jokers can be 15. you can increase this later. i know a lot of people who have gotten great results from this. you want to do this at a pace to complete under 15 minutes.