Can you walk me through the solution of: Gentry assembled a new 24 foot round pool in her backyard.  To fill the pool with water, she used four hoses.  Two of the hoses each deliver water at a rate of 2 gallons per minute.  The other two hoses each deliver water at a rate of 3 gallons per minute.  With a volume of 15,000 gallons, how many hours will it take to fill the pool?