We need to find the critical t-value at confidence level of 0.99 for the given sample size. Since the sample size is 13, the degree of freedom(df) will be 12, as:
df = Sample Size - 1
So, we need to check the critical value column under confidence level 0.99 and across the row showing df = 12. From the t-table this value comes out to be 3.055.
A picture of the t-table showing the result is attached with. Highlighted cell show the result of this question.
