The prevalence of dental caries in a community in the year 2000 and 2003 was 18% and 38% respectively. The incidence of caries for the same population in three years would be:: (A) 38%, (B) 20%, (C) 10%, (D) 56%
Answer is B. Hint:
Incidence  = No. of new cases in a specified time (38-18 = 20%)
Prevalence = Incidence x Duration.  
Incidence = Prevalence / Duration = P/D.