9.21 The Environmental Protection Agency has collected data on the LC50 (concentration killing 50% of the test animals in a specified time interval)

measurements for certain chemicals likely to be found in freshwater rivers and lakes. For a certain species of fish, the LC50 measurements

(in parts per million) for DDT in 12 experiments yielded the following:

16, 5, 21, 19, 10, 5, 8, 2, 7, 2, 4, 9 Assuming such LC50 measurements to be approximately normally distributed, estimate the true mean LC50 for DDT with confidence coefficient 0.90.