20lbs is about 9kg, and there are 7,700 calories in a kilo of fat. So assuming you don’t want to lose muscle mass and you’re not just trying to weigh in at a number (in which case you could temporarily shed some water weight) — and assuming your metabolism plays along — you’d need to run a defect of 70,000 kcal over the month, or about 2,300 a day. That seems straight up impossible if you need to lead any kind of life.