In the United States over the years 1980–2000, sulfur dioxide emissions due to the burning...

In the United States over the years 1980–2000, sulfur dioxide emissions due to the burning of fossil fuels can be approximated by the equation

where y represents the sulfur dioxide emissions (in millions of tons) for the year t, with t = 0 corresponding to 1980.

Source: This equation (and the equation in Exercise 48) were computed using data from the book Vital Signs 1999, Lester Brown et al. (New York: W. W. Norton&Co., 1999).

(a) Use a graphing utility to graph the equation y = −0.4743t + 24.086 in the viewing rectangle [0, 25, 5] by [0, 30, 5]. According to the graph, sulfur dioxide emissions are decreasing. What piece of information in the equation y = −0.4743t + 24.086 tells you this even before looking at the graph?

(b) Assuming this equation remains valid, estimate the year in which sulfur dioxide emissions in the United States might fall below 10 million tons per year. (You need to solve the inequality −0.4743t + 24.086 ≤ 10.)