, to lose 20 pounds in 30 days, you would need to burn 2000 calories more than you eat every day. Assuming a normal, relatively sedentary life, that means you can't eat anything at all for a month. With lots of exercise you could eat, but you'd basically have to exercise off every single calorie you ate, forcing your body to use reserves to power itself for the energy needed to live. It's possible, but very, very unhealthy, and your body would definitely start consuming muscle and organ tissue in addition to fat. Weight loss that rapid does damage... and doesn't necessarily get rid of your fat.
Two pounds per week is about the upper limit of what is safe and effective -- and even that level requires regular muscle-building exercise to convince your body not to break down muscle tissue.