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A sample of 555 job-seeking candidates are drug tested, and their results are categorized under four headings.

Probability measures the possibility of an event.

Mathematically, it is represented as

$P\left(A\right)=\frac{\text{Number}\text{of}\text{outcomes}\text{resulting}\text{in}A}{\text{Total}\text{number}\text{of}\text{outcomes}}$

Let A be the event that the selected person does not use drugs.

Here,the number of people who do not use drugs is the sum of the number of people who tested true negative and false positive.

The total number of persons who got tested is 555.

The probability of event A is

$$\begin{gathered} P\left(A\right)=\frac{Numberofpersonswhodonotusedrugs}{Totalnumberofpersons} \\ =\frac{25+480}{555} \\ =\frac{505}{555} \\ =0.910 \end{gathered}$$

Therefore, the probability of selecting a person who does not use drugs is 0.910.

The computed probability value appears to be a reasonable estimate of the actual proportion of adults who do not use drugs.