The numbers of women $y$ (in millions) in the civilian work force in the United States from 2000 through 2010 (see figure) can be approximated by the model $y=0.63 t+66.3,0 \leq t \leq 10,$ where $t$ represents the year, with $t=0$ corresponding to 2000 (Source: U.S. Bureau of Labor Statistics)
(GRAPH NOT COPY)
(a) Find algebraically and interpret the $y$ -intercept of the graph.
(b) According to the model, during which year did the number reach 72 million?
(c) Explain how you can solve part (b) graphically and algebraically.