During construction of a large urban drainage canal, concrete samples were taken to ascertain that the concrete produced is consistent with specifications. According to the contract agreement, the materials engineer should be have 90% confidence that the estimate of the mean slump does not deviate from the specified value by more than 5 mm? From past experience, it has been found that the parameter of produced units is normally distributed with a variance of 12 mm. How many observations should be taken in a test sample?