100 students. 10 have flue at beginning of week. 8 develop flu. What is the risk of developing flue during the course of the week?

Question

 100 students. 10 have flue at beginning of week. 8 develop flu. What is the risk of developing flue during the course of the week?

Answer 1

Well the straightforward approach would note that of the 90 students who could have gotten the flu during the week, only 8 did. So the chance of getting the flu for those 90 students was [positive outcomes / possible outcomes] = 8/90 = 0.089 or 8.9%.