According to the World Bank, at the end of 2013 ( t = 0 ) the U.S. population was 316 million and was increasing according to the following model: 0.0074 316 t P t e , where P is measured in millions of people and t is measured in years after 2013. a) Based on this model, what will be the population of the United States in 2020? b) Determine when the U.S. population will be twice what it is in 2013