Okay, here we have this:
Considering the provided information, we are going to calculate the requested probability, so we obtain the following:
Probability of an event = Favorable Events / Possible Events
Probability that it is in the third box given that it is defective = Defectives in the third box / Total defectives
Probability that it is in the third box given that it is defective = 8/ 24
Probability that it is in the third box given that it is defective = 2/6
Probability that it is in the third box given that it is defective ≈ 0.33
Probability that it is in the third box given that it is defective ≈ 33%