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The probability of a randomly selected adult in one country being infected with a certain virus is 0.005. In tests for the​ virus, blood samples from 27 people are combined. What is the probability that the combined sample tests positive for the​ virus? Is it unlikely for such a combined sample to test​ positive? Note that the combined sample tests positive if at least one person has the virus.

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Answer:

The probability is 12.66%.

This is a low probability, so it is unlikely for such a combined sample to test positive.

Step-by-step explanation:

If the probability of being infected is 0.005, the probability of not being infected is 0.995.

Then, to find the probability of at least one of the 27 people being infected P(A), we can find the complementary case: all people are not infected: P(A').

[tex]P(A') = 0.995^{27}[/tex]

[tex]P(A') = 0.8734[/tex]

Then we can find P(A) using:

[tex]P(A) + P(A') = 1[/tex]

[tex]P(A) = 1 - 0.8734[/tex]

[tex]P(A) = 0.1266 = 12.66\%[/tex]

This is a low probability, so it is unlikely for such a combined sample to test positive.

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