At time t0 a tank contains 30 oz of salt dissolved in 60 gal

At time t=0, a tank contains 30 oz of salt dissolved in 60 gal of water. Then brine containing 5 oz of salt per gallon of brine is allowed to enter the tank at a rate of 3 gal/min and the mixed solution is drained from the tank at the same rate. How much salt is in the tank after 25 min?

Tank already contains 30 oz of salt. After each minute, salt added = 3*5 = 15 oz After 25 minutes, salt in tank = 30 + 25*15 = 405 oz