Whooping cough (pertussis) is a highly contagious bacterial infection that was a major cause of childhood deaths before the development of vaccines. About 80% of unvaccinated children who are exposed to whooping cough will develop the infection, as opposed to only about 5% of vaccinated children.

A group of 20 children at a nursery school are exposed to whooping cough by playing with an infected childIf all 20 have been vaccinated, what is the mean number of new infections?