$4\mathrm{.}60$ The following are monthly actual and forecast demand levels for May through December for units of a product manufactured by the Deborah Bishop Company in Des Moines:

$\backslash$table$[[$MONTH$,$ACTUAL DEMAND,FORECAST DEMAND$],[$May$,100,100],[$June$,80,104],[$July$,110,99],[$August$,115,101],[$September$,105,104],[$October$,110,104],[$November$,125,105],[$December$,120,109]]$

What is the value of the tracking signal as of the end of December?