In an agricultural experiment, the effects of two fertilizers on
the production of oranges were measured. Eighteen randomly selected
plots of land were treated with fertilizer A. The average yield, in
pounds, was 457 with a standard deviation of 38. Twelve randomly
selected plots were treated with fertilizer B. The average yield
was 394 pounds with a standard deviation of 23. Find a 99%
confidence interval for the difference between the mean yields for
the two fertilizers. (Round down the degrees of freedom to the
nearest integer and round the final answers to two decimal
places.)
The 99% confidence interval is ( , ).