Make forecast for period $21$ using $($i$)$ three$-$period simple moving average, $($ii$)$ four$-$period

weighted moving average with the following weights: w$1=0\mathrm{.}38,$ W$2=0\mathrm{.}32,$ W$3=0\mathrm{.}2,$ W$4=$

$0\mathrm{.}1$ and iii$)$ exponential smoothing with the smoothing constant a $=0\mathrm{.}15$; use F$1=100.$

Compare the three models using MAPE, and determine the best model.