somethig like 0.05, which we look up in a table of the cumulative t-
distribution (using the nu=14 degrees of freedom line). We may have to do
this by looking up 0.95, and changing the sign of the result.
My table gives a critical t-value of -1.76 for this, which means that 95% of
the time we should see t-values greater than this. Your value is greater than
this so we cannot reject the hypotheseis that the mean is $400 from this at