In 1980, direct U.S. business investment abroad was about 13.5 billion dollars. From 1980 through 2010, that investment grew at an average annual rate of 11.24%. (a) Make an exponential model that shows the U.S. direct investment abroad A, in billions of dollars, t years after 1980. A(t) = Correct: Your answer is correct. (b) From 1980, how long did it take (in years) for U.S. investments abroad to double? (Round your answer to two decimal places.) 6.51 Correct: Your answer is correct. (c) According to the model, how long would it take (in years) from 2010 for investments abroad to double the level present in 2010? (Round your answer to two decimal places.)