Oxygen demand is a term biologists use to describe the oxygen needed by fish and other aquatic

Question:

Oxygen demand is a term biologists use to describe the oxygen needed by fish and other aquatic organisms for survival. The Environmental Protection Agency conducted a study of a wetland area in Marin County, California. In this wetland environment, the mean oxygen demand was μ = 9.9 mg/L, with 95% of the data ranging from 6.5 mg/L to 13.3 mg/L. Let x be a random variable that represents oxygen demand in this wetland environment. Assume x has a probability that is approximately normal.
(a) Use the 95% data range to estimate the standard deviation for oxygen demand.
(b) An oxygen demand below 8 mg/L indicates that some organisms in the wetland environment may be dying. What is the probability that the oxygen demand will fall below 8 mg/L?
(c) A high oxygen demand can also indicate trouble. An oxygen demand above 12 mg/L may indicate an overabundance of organisms that endanger some types of plant life. What is the probability that the oxygen demand will exceed 12 mg/L?
Distribution
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