Last year, the mean weight of potatoes produced per plant was 10.5 pounds. this year, frank installed a new watering system, so he thinks that the mean weight of the potatoes per plant will be greater than last year. he randomly selects 30 potato plants. the mean weight of the potatoes from these plants is 12.2 pounds per plant, with a standard deviation of 3.8 pounds per plant. use alpha = 0.05. what are the degrees of freedom of the t distribution?