The number of gallons of oil that can be stored in a cylindrical tank varies jointly with the height of the tank and the square of the radius of its base. If 26437.5 gallons of oil can be stored in a cylindrical tank with a height of 20 feet and a radius of 7.5 feet, find the number of gallons that can be stored in the cylindrical tank with a height of 15 feet and a radius of 6 feet.