It is known that the daily revenue of the grab-and-go deli Dirk & Dijk is normally distributed. The manager is interested in the daily performance of her deli. A sample of revenue of 20 days is collected, and the sample mean is $320. The z-values and t-values with degree of freedom (df) equal to 19 for various confidence level (1 - ?)100% are provided in the table below:

Confidence Level (%) Za/2 ta/2 (df = 19) 90 1.64 1.729 95 1.96 2.093 99 2.58 2.861