In testing the length of life of electric light bulbs. of a particular type, it is found that 12.3% of the bulbs tested fail within 800 hours and that 28.1% are still operating 1100 hours after the start of the test. Assuming that the distribution of the length of life is normal, calculate, to the nearest hour in each case, the mean, u, and the standard deviation, o, of the distribution. A light fitting takes a single bulb of this type. A packet of three bulbs is bought, to be used one after the other in this fitting. State the mean and variance of the total life of the 3 bulbs in the packet in terms of and a and calculate, to two decimal places, the probability that the total life is more than 3300 hours. Calculate the probability that all 3 bulbs have lives in excess of 1100 hours, so that again the total life is more than 3300 hours. Explain why this answer should be different from the previous (NEAB) one.