Assuming you're seeking to lose 20 lbs of FAT and not WATER, you need a caloric deficit of 17,500 per week.
If you starve yourself by eating just a few hundred calories a day, you'll lose a max of 10,000 a week. You'll also go into starvation mode and the weight loss will come to a grinding halt.
If you eat a standard diet of 2000 calories a day, you'll need to burn 2500 calories a day in exercise. For most people, that's a DAILY 4 hour bike ride at high intensity.
If you're a highly trained cyclist who simply ate too much pizza in the last few months, then yes it's definitely possible and not necessarily unhealthy. If you're out of shape looking to get in shape, there's almost zero chance it's going to happen. Sorry. Aim for 5-8 lbs a month. Even that will be difficult.