An ecological group suspects dolphins are consuming dangerous levels of arsenic in a region of the world. Dangerous levels of arsenic are classified when they reach beyond 7.0 parts per million. A random sample of 28 dolphins in the region were tested and the data is shown in the stemplot.

2|4

2|

3|2 3

3|6 7 9

4|0 1 3

4| 5 7 7

5| 0 1 1 4 4

5| 7 8

6| 3 4

6|6

7|0 1 3

7|5 6 7

Key: 2|4 = 2.4 parts per million

Part A:What proportion of the dolphins in the sample have dangerous levels of arsenic in their systems? (4 points)

Part B:Construct and interpret a 99% confidence interval for the mean arsenic levels of the dolphins if the sample of 28 dolphins has a mean arsenic level of 5.26 parts per million and standard deviation of 1.49 parts per million. (6 points)