Final answer:
The domain of the function f(x) is the set of all real numbers between and including 0 and 20, which is expressed as [0, 20].
Step-by-step explanation:
The domain of the function f(x) is the set of all possible values of x for which the function is defined. Since the function is given as a horizontal line with the equation f(x) = 10 and is defined for the interval 0 ≤ x ≤ 20, the domain of f(x) is all real numbers between and including 0 and 20. This means that the domain of f(x) is [0, 20].