Uniform probability? Here the probability function graph is a rectangle of length 20-4 (or 16) and width / height of 1/16. Note that the area under the graph from x=4 to x=20 is 1 (which is always the case when we're discussing probability functions). The area under the graph from 15 to 20 is (20-15)(1/16), or 5(1/16), or 5/16. Thus, the probability that a specimen will yield a toxic concentration level is 5/16.
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