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Which term refers to the cost that motivates an economic decision

When the amount for land is 36,000 and the amount paid for expenses is 10,000, the balance of total asset is

Alarge tank is filled to capacity with 500 gallons of pure water. brine containing 2 pounds of salt per gallon is pumped into the tank at a rate of 5 gal/min. the well-mixed solution is pumped out at the same rate. find the number a(t) of pounds of salt in the tank at time t.