(2 points) A firm that refurbishes personal computers (PCS) recently received 24 faulty computers of a similar make and model. 21 PCs have been fixed but an intern mistakenly packaged all the PCs to storage without labeling. The repair engineer will need to test the PCs one after another until he identifies the 3 PCs that are not yet fixed. Find the probability that he tested 15 computers to complete the task.