The Following Seasonal Indexes and Trend Line Were Computed from Five

Essay

The following seasonal indexes and trend line were computed from five years of quarterly sales data.
Trend line: ŷ_t = 325 + 18.5t, t = 1, 2, 3, …20. $$\begin{array} { | c c | } \hline \text { Quarter } & \text { Seasonal index } \\\hline 1 & 1.35 \\2 & 1.22 \\3 & 0.88 \\4 & 0.55 \\\hline\end{array}$$ Forecast the sales for the next four quarters.