The people who get top ranks in IIT JEE may or may not know all the concepts.It all depends on how well you perform in those 6 hrs of exam.Regarding mock exams ,generally people with good concepts get good marks.IF its a mock exam there are many people who take the exam lightly and guess them.There are may be 10% easy questions for which you can easily guess the answers.Luck factor is different for different person.Yes there some very difficult questions in IIT JEE paper that can differentiate a good conceptual guy to normal intelligent guy.
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All the best.
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