Recently Mary gave a birthday party for her daughter at which she

Hi All,

I imagine that many Test Takers would misinterpret the information in the prompt.....UNTIL they read the information in Fact 1, then they'd catch the error.

To start, we're given some information on SOME of the children who attended a party:
1) There were 8 boys who had chocolate ice cream
2) There were 9 girls who had strawberry ice cream
3) NOBODY had both flavors of ice cream
4) EVERYBODY had ice cream.

The misinterpretation is that there were only 17 children at the party. The prompt does NOT state that, so we have to keep an open mind to the idea that there were MORE children and that we don't know what type of ice cream THOSE children ate. The specific question asks for the MAXIMUM POSSIBLE number of girls who had chocolate ice cream, so THAT also implies that there were additional children beyond the 17 described.

Fact 1: 30 children attended the party.

Here we have an actual number to work with. 30 total children - the 17 mentioned in the prompt = 13 additional children. We don't know if they're boys or girls and we don't know whether they ate chocolate or strawberry ice cream. All of that is fine - the question is asking for the maximum POSSIBLE number of girls who COULD have had chocolate ice cream. IF those 13 extra children were ALL girls and IF they ALL had chocolate ice cream.....then that would make the maximum possible = 13.
Fact 1 is SUFFICIENT.

Fact 2: Fewer than half the children had strawberry ice cream.

Since we know that 9 had strawberry and 8 had chocolate, this Fact also implies that there were additional children. Unfortunately, we don't know how many. There would have to be AT LEAST 2 extra children, but it could be dozens or hundreds and we have no idea about whether they're male or female or what type of ice cream each ate.
Fact 2 is INSUFFICIENT.

Final Answer:

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