A shopping mall manager creates a parabola to predict the number of shoppers in a mall, where $x$ is the number of hours since 8 A. M. And $y$ is the number of shoppers in the mall. Write a function $f$ that models the number of shoppers in the mall over time. Then find the average rate of change in the number of shoppers over the interval in which the number of shoppers is increasing. Round your answer to the nearest whole number.