The Trend Line $\hat { y }$ = 500 + 30t, (T = 1,2,3

Essay

The trend line $\hat { y }$ = 500 + 30t, (t = 1,2,3,.... ,20),and the seasonal indexes shown in the table below were computed from five years of quarterly sales data.Forecast the sales for the next four quarters.
$$\begin{array} { | c | c | } \hline \text { Quarter } & \text { Seasanal Index } \\\hline 1 & 1.4 \\2 & 1.2 \\3 & 0.9 \\4 & 0.5 \\\hline\end{array}$$