Due to the central limit theorem, we know that a sample mean, ybar, has an approximate normal distribution. In this problem you are given that the variance of this distribution is 200^2. Hence:

(ybar - mu)/200

where mu is the population mean acreage, has an approximate standard normal distribution.

The probability that the sample mean ybar falls within 10 acres of the population mean acreage is:

P(|ybar-mu| < 10) =

P(ybar-mu < 10) - P(ybar-mu < -10) =

P((ybar - mu)/200 < 1/20) - P((ybar - mu)/200 < -1/20) =

P(z < 1/20) - P(z < -1/20)

where z is a standard normal random variable. You can look these last probabilities up in the table in the back of you stat book.

~Matt