A tank initially contains a solution of 10 pounds of salt in 60 gallons of water. Water with 1/2 pound of salt per gallon is added to the tank at 6gal/min, and the resulting solution leaves at the same rate. Let A(t) be the amount of salt in the tank after 7 minutes. Set up the differential equation that models the rate of change of salt in the tank, and give any initial values.