Hi gdshamain,

Certain DS questions start off "thin", meaning that there is not much information (or clearly not enough information) to answer the given question. It's in the last couple of "steps" that you have to put in the real work.

Here, we're told that at a 2-day seminar, 90% of those registered attended the seminar on the 1st day. We're asked what percent of those registered did NOT ATTEND on either day.

Fact 1: Total registered = 1,000 people

This tells us that 900 people attend on the 1st day (and 100 did not), but it does not tell us anything about the 2nd day.
Fact 1 is INSUFFICIENT

Fact 2: Of those registered, 80% attended on the 2nd day.

Wth the information offered in the prompt, there are some possibilities to consider:

1) It's possible all 80% of the 2nd day attendees also attended on the 1st day; this means that 10% did not attend either day.
2) It's possible that NOT all 80% of the 2nd day attendees also attended on the 1st day; this means that some percent OTHER THAN 10% did not attend either day.
Fact 2 is INSUFFICIENT.

Combined, we run into the same problem that we did in Fact 2: even though we know that there were 1000 people registered, we DON'T KNOW how many of the 2nd day attendees attended on the 1st day.
Combined, INSUFFICIENT.

Final Answer: E

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