When there is less being spent than what is produced, production decreases since less output is needed to meet demand.. If a=5, I=500 G=500, X=500, T=1000, mpc= 0.5 and mpm=.4, then calc the value of the export multiplier. Of course, in this very simple model, I've ignored a few things like inflationary effects, short and medium run effects because this is very introductory..
