Depending on your body structure and lifestyle, first find out what is your basal metabolic rate (BMR) and daily calorie needs (there are many online calculators available for this or your nutritional expert can guide you regarding this). This is the total calories you need to consume per day. To lose about one pound, you need to burn about 3500 calories per day. So multiply this by 20 pounds. Then calculate the calorie you need to save per day by dividing this number by 75 (2 ½ months). This is the amount of calories you could eat less per day from your daily calorie requirements. This coupled with exercise of any kind (walking, jogging, swimming, tennis etc) will help you lose weight faster and allow you to eat some more. Take care!
The medical advice given should not be considered a substitute for medical care provided by a doctor who can examine you. The advice may not be completely correct for you as the doctor cannot examine you and does not know your complete medical history. Hence this reply to your post should only be considered as a guiding line and you must consult your doctor at the earliest for your medical problem.