MEDICAL RESEARCH Based on data obtained from the National Institute of Dental Research, it has been determined that 42% of 12-yr-olds have never had a cavity, 34% of 13-yr-olds have never had a cavity, and 28% of 14-yrolds have never had a cavity. Suppose a child is selected at random from a group of 24 junior high school students that includes six 12-yr-olds, eight 13-yr-olds, and ten 14-yrolds. If this child does not have a cavity, what is the probability that this child is 14 yrs old?