Hi The question is: In measuring the time people take to react to a flashing light, it is estimated from a large number of previous experiments that the standard deviation is 0.5 seconds. How many observations must be taken in a new experiment in order to be 99% confident that the error in the estimate of the mean reaction time will not exceed 0.1 seconds? The answer is 166 but im not quite sure how this is obtained, any ideas?