Final answer:
The function represented by a horizontal line does not have an interval of increase because its slope is zero, indicating no change over the domain 0 ≤ x ≤ 20.
Step-by-step explanation:
The question pertains to identifying intervals of the increase for a given mathematical function. The function in question appears to be written with a typographical error and lacks clarity. However, we were provided with additional reference information indicating the graph of a certain function f(x) is a horizontal line for the domain 0 ≤ x ≤ 20. A horizontal line has a slope of zero, which means the function does not increase or decrease; it remains constant. Therefore, the function does not have an interval where it is increasing since its rate of change is zero throughout the specified domain.