If you don't want to gain muscle mass and simply lose weight in a healthy way, then it's pretty simple: 1. Do tons of cardio be it running, cycling, swimming or rope skipping. it doesn't have to be all of those, you can choose to simply run everyday but if you mix it up a bit it might avoid you burning out. This is how you will lose weight without putting on muscle mass, by doing cardio. If you do workouts that work your arms, core, back etc. you could potentially put on muscle mass especially if it involves weights. 2. Stick to eating lots of vegetables, and some fruits, fish and poultry (chicken & turkey). No sugar, no processed foods, no alcohol, no read meat. 3. Drink a lot of water everyday until a couple of days before the final weigh in where you can reduce the amount you drink a bit. Losing 20 pounds in 3 weeks is not crazy depending on how fat or skinny you are already.