A simple forecasting method for weekly sales of flash drives used by a local computer dealer is to form the average of the two most recent sales figures. Suppose sales for the drives for the past $12$ weeks were

$\backslash$table$[[$Week:$,1,2,3,4,5,6,7,8,9,10,11,12],[$Sales:$,86,75,72,83,132,65,110,90,67,92,98,73]]$

a$.$ Determine the one$-$step$-$ahead forecasts made for periods $3$ through $12$ using this method.

$b.$ Determine the forecast errors for these periods.

c$.$ Compute the MAD, the MSE, and the MAPE based on the forecast errors computed in part $(b).$