among the patients of a certain dentist, 25% are children under 18, 30% are aged 18-30, and 45%are over 30. Past experience shows that the probability of having a cavity is .9 for a child under 18, .4...

among the patients of a certain dentist, 25% are children under 18, 30% are aged 18-30, and 45%are over 30. Past experience shows that the probability of having a cavity is .9 for a child under 18, .4 for a person aged 18-30, and .2 for someone over 30. If a patient is selected at random, find the probability that they will not have a cavity