Totally legitimate question.
If the target question is "
Is this triangle isosceles?," then a statement will be sufficient if we can answer with EITHER yes OR no
with absolute certainty.
That is, if we can say, with certainty, "
yes, this triangle is isosceles," then the statement is sufficient.
Likewise, if we can say, with certainty, "
no, this triangle is not isosceles," then the statement is sufficient.
Here's a different example:
Target question:
Is x an even integer?
Statement 1: 2x + 1 = 13
When we solve the equation, we get x = 6, so we can say, with certainty, "
yes, x is an even integer.
So, statement 1 is sufficient.
Another example:
Target question:
Is x an even integer?
Statement 1: 2x + 1 = 11
When we solve the equation, we get x = 5, so we can say, with certainty, "
no, x is not an even integer.
So, statement 1 is sufficient.
It's all about whether you can definitively answer the target question. It doesn't matter whether th actual answer is yes or no.
NOTE:
Everyone struggles with Data Sufficiency (DS) questions at first. This question type is totally unique to the GMAT, so it's foreign territory.
If you're looking for some extra DS resources, we have a free set of videos that cover everything you need to know:
https://www.gmatprepnow.com/module/gmat-data-sufficiency
Cheers,
Brent
