A pet store has a large tank in the shape of an inverted right circular cone that they use to display fish to their customers. Today, the tank needs to be drained in order to clean it. The radius of the cone at its top is 150 cm and its depth is 90 cm. Water is flowing out of the tank at a rate of 3 liters per second. How fast is the depth of the water in the tank decreasing when the water is 60 cm deep? (Note: 1 mL = 1 cubic centimetre)