3-20. A company that manufactures shuttle cars and mines for the coal mining industry has determined that “coal production in the U.S.” is a fairly reliable three-quarter leading indicator of their sales. This quarter’s coal production in the United States can be obtained from the Department of Energy and it is used to obtain a forecast of their shuttle car and miner sales nine months from now. For the data below, determine a linear equation based on regression analysis and prepare a forecast of company sales three quarters from now. (3.4.3) Quarter t Future Sales Coal Production in (units/quarter) the United States yt+3 (103 tons/quarter) xt 1 341 173 2 355 147 3 248 106 4 406 184 5 421 233 6 375 169 7 341 210 8 267 121 9 289 118 10 489 271 11 (now) ? 198