A sample of 18 boxes from a packaging line have their length measured. The mean is 29.27 cm with a standard deviation of 2.10 cm. When constructing a confidence interval for the population mean of the lengths, find:

a) What distribution should be used (Type z or t) [1 mark]

b) What is the critical value? [2 marks]

c) What is the 90% confidence interval for the mean of lengths. Type the limits of the interval in order (first the left one, then the right one separated by a comma - no spaces). Express your values correct to 2 decimal places. [2 marks]