: A tank contains salt dissolved in 120 gallons of water. A solution containing 1/2 pound of salt per gallon is pumped into the tank at a rate of 6 gallons per minute. At the same time, the tank is drained at a rate of 6 gallons per minute. Let y be the number of pounds of salt in the tank at time t, measured in minutes. (i) Write a differential equation modeling the scenario described above. (Note: no work required)