To detect a heart malformation, a rapid test is used that fails in 10% of sick people and 20% of healthy people. When this test is positive, a second test is done in the hospital, with more precise equipment. In this case, if the person is healthy, the second test is not wrong, and if he has the heart malformation, it fails 1% of the time.
Knowing that the incidence of this malformation is 15 cases per 1000 inhabitants, it is requested:

(a) What proportion of investigated people have a negative result in the second test?
(b) If a person has a negative result in the second test, what is the probability that he or she has that malformation?