申論題

4.(10 points) When one person is infected with c-virus and interacts with another one who is healthy, there is a probabilityp = 0.2 to get the healthy one infected. Two people in the community are selected randomly at any single period of time and interact with each other. Let x_n be the number of infective people at time n and there are five people in the community. Given x₀ = 1, what is the mean time to infect everyone in the community?