Class 9th ganit Manjari chapter 2 introduction to linear polynomials exercise 2.3 question 2

Class 9th ganit Manjari chapter 2 introduction to linear polynomials exercise 2.3 2nd question a really starts with 120 members each hour 9 members dropout of the group how many members will remain after 1 2 3 hours find the linear expression to represent the number of members at the end of the nth hour.