Textbook sales. The quarterly sales data (number of copies sold) for a college textbook over the past 3 years are as follows:

Quarter	Yr 1	Yr 2	Yr 3
1	1690	1800	1850
2	940	900	1100
3	2625	2900	2930
4	2500	2360	2615

d. Let t= 1 refer to the observation in quarter 1 of year 1; t=2 refer to the observation in quarter 2 of year 1;.; and t=12 refer to observation in quarter 4 of year 3.
Using dummy variables defined in part (b) and t, develop an equation to account for seasonal effects and any linear trend in the time series.

e. Based upon the seasonal effects in the data and linear trend, compute the quarterly forecasts for the next year.