Answer: Expected number of tests = 5.013
Step-by-step explanation:
Define random variable X that marks the number of tests required for some certain
group. Observe that if the test is negative for all the people (which has probability 0.95), we make one and only one test. If some of the people is tested positive, we make ten additional tests for every person in that group separately. Hence, the expected number of tests will be for if they are all negative (1 test) and the case of at least one person testing positive (11 tests).
That Is,
E(X) = 1(0.95^10) + 11(1 - (0.95^10))
E(X) = 0.5987 + 4.414 = 5.013