In order to determine the average weight of carry-on luggage by passengers on a particular airplane, a sample of 22 pieces of carry-on luggage was weighed. The average weight was 9 kg. It is known that the weight of carry-on luggage is normally distributed and that historical data has shown the standard deviation is 3.45 kg. Determine the critical value for the test statistic (z or t) needed to build an 80% confidence interval for the mean. State the positive value exactly as found from the tables (2 decimal places).