Incongruent test

We note that $2400$ people have the virus, and $3600$ do not. For each person who has the virus, the probability that the results are incongruent is $\dfrac{19}{20} \dfrac{1}{20} + \dfrac{1}{20} \dfrac{19}{20}= \dfrac{19}{200}$ . This is true because we need both a positive and a negative result in order for our results to be incongruent, and these can happen in any order. Thus, we expect that about $2400 \cdot \dfrac{19}{200} = 228$ of these will have the virus. Similarly, for those without the virus, the probability of incongruent results is $\dfrac{9}{10} \dfrac{1}{10} + \dfrac{1}{10} \dfrac{9}{10}= \dfrac{9}{50}$ , and since there are $3600$ of these, we expect incongruent results on $3600 \cdot \dfrac{9}{50} = 648$ of these. Thus, we expect a total of $648+228=\boxed{876}$ people to have incongruent results.