Answer:
[tex]5/8[/tex]
Step-by-step explanation:
Since all the people sit on the chairs at random, the probability of a certain chair being occupied by a boy is equal to the probability of randomly picking a boy from the class. Since there are 24 people total and 15 of them are boys, there is a [tex]\frac{15}{24}=\boxed{\frac{5}{8}}[/tex] chance that the thirteenth chair is occupied by a boy.