The following represents demand for widgets (a fictional product):
QD = 650 - 4P - 0.004M + 4PR
where P is the price of widgets, M is income, and PR is the price of a related (fictional) good, the widget. Supply of widgets is determined by QS = 20 + 4P
a. Determine whether widgets are a normal or inferior good, and whether widgets and wodgets are substitutes or complements.