A carload of stone taken from a quarry by the plug-and-feather method contains pieces of stone have a mean weight of 1,500 pounds and a total weight of 36,000 pounds. Another carload of stone produced by the explosive method (where stone is blasted into random shapes by explosives) contains pieces of stone that have a mean weight of 600 pounds and a total weight of 30,000 pounds. What is the mean weight of all the pieces of stone combined?
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