Not sure what "ASAP" means to you, but usually dropping weight ASAP runs contrary to preserving muscle. Anyway, it's just simple math really, 1 pound = 3,500 calories. Let's say ASAP means losing one pound a week, using Venomancer's 21 pound number, it'd take 21 weeks of cutting (or burning) 500 calories a day. This would certainly preserve high percentage of muscle mass. On the other hand, let's say ASAP means means losing 5 pounds a week, half fat/half muscle ( although I have no idea what YOUR exact fat/muscle ratio would be) which means cutting (or burning) 2,500 calories a day for 4 weeks, or so.
You just have to pick a desired weight loss per week, figure out how many calories you need to either cut from your diet or burn doing cardio, and away you go. I tend to think you could lose 1-2 pounds a week without hurting your muscle mass too much, but I doubt that's the speed at which you're hoping to lose
Of course the math is swayed by supplement use but someone else will have to speak to that.
Best of luck.