Montana imposed a severance tax on every ton of coal mined within the state. The tax varied depending on the value of the coal and the cost of production. It could be as high as 30 percent of the price at which the coal was sold. Montana mine operators and some out-of-state customers claimed that this tax was unconstitutional as an improper burden on interstate commerce. Decide. [Commonwealth Edison Co. v Montana, 453 US 609]