The amount of vacation hours he has can be modeled with the equation V = .25w + 10, in which V is equal to vacation days and d is equal to the number of weeks he worked.
We can tell this because he gets 10 days off no matter what. This is called a constant and is always added to the end of the equation.
We can also tell this because the number of weeks he worked gets multiplied by a quarter of a day. Therefore, he gets a vacation day for every 4 weeks he works.
This model will successfully tell you how many he has at the end of the year.