That is an unhealthy goal to have. What height are you?
Forget the number on the scale and focus on being healthy. Do you eat much on an average day?
I don't know if you have an ED or not but we just want you to be healthy and clear of diet fads, crazily low weight goals etc.. 80lbs sounds a little on the dangerous side, if you do have an ED, focus on getting that tended to by a professional.