15.23 Naa, you got it wrong, your getting very confused. If someone coughs 20 meters away from where your standing, and their not wearing a face mask, then the finer droplets can travel up to 20 meters, towards you, if you have a mask on there's a very good chance they will stop the droplets enter your nose and mouth. If that same person wears a mask, then there's less chance of it spreading to 20 meters. So to conclude, if both wear a mask then there's less chance of the virus spreading. Hope this helps, its just common sense really. Oh, and why do you think care homes were crying out for masks for the last 4 months?