Per capita income the per capita personal income in dollars

Question

Question: Per capita income the per capita personal income (in dollars per year) in the United States for selected years from 1960 and projected to 2018 can be modeled by

I(t) = 13.93t² + 136.8t + 1971

where t is the number of years past 1960 (Source: U.S. Bureau of Labor Statistics). Use n = 10 equal subdivisions with right-hand endpoints to approximate the area under the graph of I(t) between t = 50 and t = 55. What does this area represent?