The Following Seasonal Indexes and Linear Trend Were Computed from Five

Essay

The following seasonal indexes and linear trend were computed from five year quarterly sales data.
Trend line is $$\hat { y } = 500 + 30 t , ( t = 1,2,3 , \ldots .20 )$$ $$\begin{array} { | c | c } \hline \text { Quarter } & \text { Sea50nal Index } \\\hline 1 & 1.4 \\\hline 2 & 1.2 \\\hline 3 & 0.9 \\\hline 4 & 0.5 \\\hline\end{array}$$ Forecast the sales for the next four quarters