7. Population decay. Since 1990, the population of Gary, Indiana, has been decreasing by 1.87% per year. The rate of change of the city's population P, t years after 1990, is given by dP dt = -0.0187P. a) In 1990, the population of Gary was 116,646. (Source: U.S. Census Bureau.) Find an exponential function that models this situation. b) Estimate the population of Gary in 2025. c) What is the rate of change of the population of Gary in 2025? d) After how many years will the population of Gary be half of what it was in 1990?