If Juan had a doctor's appointment on a certain day, was the appointment on a Wednesday?

There's not much we can do to rephrase the question stem, so all we can do is try to prove the statements INSUFFICIENT.

1) Exactly 60 hours before the appointment, it was Monday.

Monday at 12:00am + 60 hrs = 12:00pm on Wednesday
Monday at 11:59pm + 60 hrs = 11:59am on Thursday

Maybe the appointment is on Wednesday, but maybe it isn't. INSUFFICIENT.

2) The appointment was between 1:00 pm and 9:00 pm

This doesn't indicate what day the appointment was on. INSUFFICIENT.

Together:
If the appointment was between 1:00pm and 9:00pm, and 60 hrs before it was Monday, then the appointment has to be on Wednesday. As we've seen, Monday + 60 hrs only gives us a possible range of noon on Wednesday until 11:59am on Thursday. If the appointment was afternoon/evening, it had to be on Wednesday.

The answer is C.

Hi BlueDragon2010,

The other posts in this thread have properly explained the "math" behind this question, so I won't rehash that here. Instead, I'll offer some perspective on how DS questions are written. In many Quant questions, you'll be asked to solve for "one thing" (a variable, total, speed, probability, etc.); while many DS questions will ask you for a value, others will ask you a "yes/no" question. While the tactics for solving these types of questions is similar to other questions, your focus needs to be on the "consistency" of the results (and whether the results stay the same OR change). This question asks WAS the appointment on a Wednesday (a yes/no question)? MAYBE the appointment was on a Wednesday, MAYBE it wasn't. In these situations, make sure to be thorough with the possibilities and consider the options. Those considerations are the only way to be sure that you're answering the question correctly.

GMAT assassins aren't born, they're made,
Rich