Answer
95% Confidence Interval: $342,426\lt x̅\lt420,389$
Work Step by Step
In MINITAB, with the results from item (c) still available:
Select Stat -> Basic Statistics -> 1-Sample t
Choose "One or more samples, each in a column". In the box bellow enter C1
Click in Options. Enter 95 in Confidence level. Select Mean $\ne$ hypothesized mean. Click Ok.
Click Ok.
