18.

The sales of a Gnagey's Groceries average $8,000 per day. Gnagey introduced several advertising campaigns in order to increase sales. To determine whether or not the advertising campaigns have been effective in increasing sales, a sample of 55 days of sales was selected. It was found that the average was $8,240 per day with a sample standard deviation of $1,221. The null hypothesis is to be tested at 0.01 significance level.

The critical value is

(round the final answer to two decimal places, e.g. 2.57)

(There are two answers that will be accepted as correct for this question)