4. (25 pts) A mixture of methane and air is capable of being ignited only if the mole percent of methane is between 5% and 15%. A mixture containing 12.0 mole% methane in air flowing at a rate of 6.00*102 kg/h is to be diluted with pure air to reduce the methane concentration to the lower flammability limit. Calculate the required flow rate of air in mol/h and the percent by mass of oxygen in the product gas. (Note: Air may be taken to consist of 21 mole%O2 and 79% N2 and to have an average molecular weight of 29.0.)