A restaurant sells tea for $1.50 plus $0.50 per refill. the restaurant brews enough tea for 4 refills per customer. The linear function that represents the total cost of, r , tea refill is C(r)=0.5r+1.5.

C(r)=0.5r+1.5.
The domain of a function is the set of all input variables of the function. In this case "r"
The domain r is the number of tea refills.