Concept:
A Yes/No is an answer to the Question Stem in a DS question. 
If by using plug in values you get two answers and hence no definite answer like a confirmed Yes or a confirmed No, the statement stands insufficient.

So say the question stem is 
Is x+ y >5   ? for
Statement  x>3 
It can have a yes for x=4 and y=3 (x+y=7>5)

and a No for x=4 and y=0 (x+y=4 and thus NOT >5) 

Thus, there is NO definite answer.=>Insufficient.