4) The lifetime of light bulbs manufactured by an electrical company is approximately normally distributed with a standard deviation of 50 hours. The average lifetime of a sample of 25 bulbs has been measured as 750 hours- a) What is the 90% condence interval of average lifetime of lightbulbs manufactured by this company? b) How large of a sample is needed to be 96% condent that the sample mean will be within 12 hours of the actual mean? (9 marks)