The daily total cost C equals the fixed cost of $3,000 plus the cost of producing n units.

Since the cost of producing 1 unit is $4, the cost of producing n units is $4n

Thus the total cost C, where C = f(n) is

C = 3000 + 4n

Here the domain is "all integer values, n ≥ 0" while the range is "all integer values, C ≥ 3000"