Example $18$

Part$-$time students admitted to an MBA program in a university are considered to be first$-$year students until they com plete $15$ credits successfully. Then they are classified as second$-$year students and may begi n to take more advanced courses and to work on the thesis required for graduation. Past record indicate that at the end of each year $10\%$ of the first$-$year students $($F$)$ drop out of the program $($D$)$ and $30\%$ become second$-$year students $($S$).$ Also, $10\%$ of the second$-$y ear students drop out of the program and graduate $($G$)40\%$ each year. Students that gradu ate or drop ou