Final answer:
The domain of the function described is all real numbers from 0 to 20, inclusive, which is expressed in interval notation as [0, 20].
Step-by-step explanation:
The domain of a function refers to the set of all possible input values (usually represented as 'x') for which the function is defined. In the case of the function you described, if the graph of f(x) is a horizontal line and f(x) is defined for 0 ≤ x ≤ 20, then the domain of the function is simply all real numbers from 0 to 20, inclusive.
This means that you can plug in any real number between 0 and 20 into the function f(x), and the function will have a value.
To express this in interval notation, we write the domain as [0, 20]. The square brackets indicate that the end values 0 and 20 are included in the domain, which corresponds to the function being defined at these endpoints as well.