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Data are given in 4 categories depending on the true condition versus the test results for a sample of 555 people.

Probability is the number of outcomes that result in an event (E) divided by the total number of outcomes.

It is computed using the formula

$P\left(E\right)=\frac{\text{Number}\text{of}\text{favourable}\text{outcomes}\text{of E}}{\text{Total}\text{number}\text{of}\text{outcomes}}$

Let A be the event of selecting a person who got a false negative result.

The total number of persons who were tested is 555.

The number of persons who got a false negative test result is 5.

The probability of event A is

$$\begin{gathered} P\left(A\right)=\frac{Numberofpersonswhotestedfalsenegative}{Totalnumberofpersons} \\ =\frac{5}{555} \\ =0.00901 \end{gathered}$$

Therefore, the probability of selecting a person who got a false negative test result is 0.00901.

It can be concluded that the employer will suffer from a false negative test result.

The randomly selected person would test negative for the use of drugs when he actually uses them.

For such a subject, the employer would be at the risk of hiring a person who continues to use drugs.