In an experiment to assess the strength of the hunger drive in rats, 40 previously trained animals were deprived of food for 24 hours. At the end of the 24-hour period, each animal was put into a cage where food was dispensed if the animal pressed a lever. The length of time the animal continued pressing the bar (although receiving no food) was recorded for each animal. The data yielded a sample mean of 19.1 minutes with a standard deviation of 5.4 minutes.

Required:
Estimate the true mean time and calculate the margin of error.