Lifetime tests for a sample of 51 tungsten light bulbs taken from a large batch are carried out, and the mean lifetime for the sample is found to be 984 hours, with a standard deviation of 9.4 hours....

Lifetime tests for a sample of 51 tungsten light bulbs taken from a large batch are

carried out, and the mean lifetime for the sample is found to be 984 hours, with a

standard deviation of 9.4 hours. Calculate the true standard deviation for the whole

batch of light bulbs from which the sample of 51 was taken to (a) a 95% significance

level and (b) a 99% significance level.