Four identical districts of a mine are ventilated with a quantity of 3500 m³/min at a fan drift pressure of 1.15 kPa. When one of the districts is sealed off, the change in resultant resistance is 0.072 Ns²/m⁻⁸. If the fan is stopped, keeping a district sealed, the quantity through the mine becomes 850 m³/min. The natural ventilation pressure in Pa, is