Confidence Interval for the Mean μ
The t distribution is symmetric but there is a different distribution for each sample size.The t distribution is symmetric but there is a different distribution for each sample size.
The critical value of t can be found in Table A-3 by looking in the column for the area (significance level) in two tails and the row for the number of degrees of freedom. df = n - 1 
and the standard deviation, s of the sample if they are not given in the problem.