31. Refer to the previous exercise. A sample is selected of the size believed to be needed to estimate the mean correct to within 25 acres with probability .95 Suppose, however, that the sample has a standard deviation of 400 acres, rather than 200. Then, how close can we expect the sample mean to be to the true mean? In other words. what amount is added to and subtracted from the sample mean to get a 95% confidence interval for the mean acreage of farms?