Given the prevalence, sensitivity and specificity of a single test are known.
1 Answer
You only need one of the three measures
prevalence the proportion of the population that has a certain condition
sensitivity the probability that a unit that is truly positive will test positive.
In simple words: 'how well does the test detect positive cases'
specificity the probability that a unit that is truly negative will test negative.
In simple words: 'how well does the test ignore negative cases'.
Your question relates to the probability that a people are infected (in a sample of size 'k'). Which quantity do you need to compute this? Hint: the sensitivity and specificity relate to the situation when there has been a test, your question does not mention anything about test results.