Hi aditiniyer,

To start, you should have posted this prompt in the DS Forum AND you should have included the correct answer to the question.

As it stands, this question can be answered with 'logic' (and not a lot of math is needed). We're asked for the PERCENT of students at this School who study French but NOT Spanish.

1) 30% of all students at Jefferson High School study French.

With this Fact, we have no idea what percent of the total (nor of this sub-group) don't study Spanish. We do know that the answer to the question would be no more than 30% though.
Fact 1 is INSUFFICIENT

2) 40% of all students at Jefferson High School do not study Spanish.

With this Fact, we have no idea what percent of the total (nor of this sub-group) study French. We do know that the answer to the question would be no more than 40% though.
Fact 2 is INSUFFICIENT

Combined, we have no way of knowing how the two groups described in the two Facts "overlap." It could be that EVERY student described in Fact 1 is ALSO in the group described in Fact 2. However, it's possible that far less than "every" student is in that second group (and it's possible that NONE of them are.
Combined, INSUFFICIENT.

Final Answer: E

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