1) Initially, a large tank with a capacity of 300 gallons contains 150 gallons of clean water. A salt solution with a concentration of 3 lb/gal flows into the tank at a rate of 30 gal/min. The solution is perfectly mixed while it is extracted at a rate of 15gal/min. Find: 1) the amount of salt in the tank at the time it is filled (in pounds). 2) The speed at which salt is coming out at that instant (in pounds per minute). 3) The amount of salt that has left the tank from the beginning and until that time (in pounds).