In a population of 5000, incidence of disease is 100  in 1 year duration of disease studied for is 2 years calculate Prevalence -
Incidence = No. of disease person / Population at risk x 1000 = 100/5000 = 1000 = 20 per one thousand per year 
Prevalence = I x D = 20 x 2 = 40 per one second
20/1000
40/1000
80/1000
400/1000