A study proposed in 1980 by researchers from the major producers and consumers of the world's coal concluded that coal could and must play an important role in fueling global economic growth over the next 20 years. The world production of coal in 1980 was 3.5 billion metric tons. If output increased at the rate of 3.Se billion metric tons/year in year (t-O corresponding to 1980), determine how much coal was produced worldwide between 1980 and 2000.