Can someone please explain it to me ASAP??!!!

A 500-gal tank initially contains 100 gal of brine containing 75 lb of salt. Brine containing 1 lb of salt per gallon enters the tank at a rate of 5 gal/s, and the well-mixed brine in the tank flows out at the rate of 3 gal/s. How much salt will the tank contain when it is full of brine? The tank will contain of salt when it is full of brine. (Type an integer or decimal rounded to two decimal places as needed.)