Is it normal to lose 40 pounds in 20 days?

Normal. According to the control of exercise and diet, you can lose 0.5-3 kg a day, 20×0.5= 10 kg 20×3=60 kg for 20 days, and 40 kg for 20 days belongs to the normal range, which requires 4 10-5 10 calories for daily exercise, so it is normal.