You don’t actually expect people to answer all that right
It makes readers think about how long Ellery usually takes to solve a case.
Explanation:
The insistence of one person being worried and looking at the clock is that the investigators are running late and it could be due to a variety of reasons.
They could be in grave danger or there is an imminent threat of something else happening around them as they focus on this case.
The other person tells him smugly that the detective still has 28 minutes to solve the case.
This sentence insinuates that the detective usually only takes that much time to solve the case, showing the speaker's confidence in his abilities.
The device would be a metaphor because the phrase is comparing headmasters to giants without using the words like or as
