Find the answer to the math problem: This morning, farmer Rita's workers collected 30,000 gallons of milk and chilled it in a storage tank. They then spent the next 4 hours pumping milk at a rate of 2,880 gallons/hour from the storage tank into a milk tanker. For the next 7 hours, the workers added more milk into the storage tank, at a rate of 1,500 gallons per hour. How many gallons of milk were left in the storage tank?