In 2010 number of people infected by the virus was Po. Due to new vaccine, number of infected people has reduced by 20% each year since 2010. In other words, only 80% as many people are infected each year as were infected year before. Determine formula for P = f(n), number of infected people n years after 2010. Graph f(n). Describe, in terms of virus, why graph has shape it does.