Hello

,
20 pounds in 8 weeks is ambitious, but definitely possible. The best way to achieve this goal, by natural means, is to get a lot of exercise with cardio and eat healthy.
I would recommend strength training at least 3 times a week... (on opposite days) and then do interval training where you are going fast, and then slow down or you are going at a higher exertion and then slow down 45 minutes 3-4 times a week and the the rest of the week do cardio and work out slower.... so you have a good cardio workout at least 7 times a week and strength training 3 times.
Also eating healthy is very important. Get rid of the fats, the carbs, the sugars, juices, fatty dairy and eat a lot of veggies and fruits and make sure not to forget protein.. if you dont eat protein or enough calories.. you will lose lean muscle tissue which will work against you.
Drinking lots of water will also help naturally increase your metabolism and curb your appetite.
Good luck. Let us know how it works out.