Final answer:
The function described, f(x) = 20, is a horizontal line within the interval [0, 20] and does not increase or decrease, hence there is no interval on which the function is increasing.
Step-by-step explanation:
The question concerns determining the largest interval on which a given function is increasing. For the function described, f(x) = 20 for all x in the closed interval [0, 20], the graph of this function would be a horizontal line. Since a horizontal line has a slope of zero, it is neither increasing nor decreasing, but rather constant. Therefore, there is no interval on which this function is increasing, because the rate of change of the function with respect to x is zero across the entire domain given.