What price do farmers get for their watermelon crops? In the third week of July, a random sample of 40 farming regions gave a sample mean of  = $6.88 per 100 pounds of watermelons. Assume that σ is known to be $1.92 per 100 pounds.
(a) Find a 90% confidence interval for the population mean price (per 100 pounds) that farmers in this region get for their watermelon crop. What is the margin of error?
(b) Find the sample size necessary for a 90% confidence level with maximal margin of error E = 0.3 for the mean price per 100 pounds of watermelons.
(c) A farmer brings 15 tons of watermelons to market. Find a 90% confidence interval for the population mean cash value of this crop. What is the margin of error?