The weekly demand for sulphuric acid from the store of a chemical factory is normally distributed with mean 246 litres and standard deviation 50 litres. After placing an order with the sulphuric acid manufacturers, delivery to the store takes one week. (a) How low can the stock of sulphuric acid be allowed to fall before ordering a new supply in order to be 95% sure of meeting all requirements while waiting for the new supply to arrive? (b) What volume of sulphuric acid should then be ordered so that, with 95% certainty, it will not be necessary to reorder within 6 weeks?