An agricultural researcher is interested in planting a new variety of yellow corn, whose yield follows a Normal distribution with an unknown mean and a standard deviation of -10 bushels per acre. They plant 25 plots of the corn and get an average yield of x-150 bushels per acre. (a) What is the 90% confidence interval? (b) How many plots would need to be planted to reduce the margin of error to 2.5 at the 90% confidence level?