The economic dependency ratio is defined as the number of persons in the total population who are not in the workforce per 100 in the workforce. Since 1960, Baby Boomers in the workforce and a decrease in the birth rate have caused a significant decrease in the economic dependency ratio. With data for selected years from 1960 and projected to 2050, the economic dependency ratio R can be modeled by the function R(x) = - 0.0002x3 + 0.052x2 - 4.06x +192 Where x is the number of years past 1950 (Source: U.S. Department of Labor). Use this model to find the year in which the economic dependency ratio reached its minimum. What was happening in the United States around this time that helps explain why the minimum occurred in this year?